[RPG] What mount should I try to get

You can not have a Warhorse as beast companion.

Beast Mastery: You gain a beast companion, chosen from one of these categories: bear, boar, cat, lizard, raptor, serpent, spider, wolf or horse. (Martial Power page 38 + Dragon 392)

A Horse Beast Companion is not a Warhorse in any way concering rules. Your DM could houserule a Warhorse is a horse, but I would advise against it. It would be overpowered, as a Warhorse is quite more powerful than any of the allowed types. (HP, trample, defenses, attack, damage)

The horse as a Beast Companion is weaker than a Warhorse for balance reasons. Changing HP and Defenses is not enough, change damage too, for trample as well.

Trample and Charger could be added at the cost of one feat each if they are usable at-will, or both for one feat if usable once per encounter. Still, trample should do 1d8 damage.

I would give you a free Saddle of Strength, it takes up the slot of the much better Impenetrable Barding anyway.

You need the Beast Rider feat for it just like with any other Beast Companion. For this reason and because of the better feat support, I would still go with a refluffed Bear.

Mark of Handling gives feat bonus to mounts, and feat bonus to beast companions. Feat bonuses do not stack, so no, it is not +4 speed +2 AC.

The nightmare is the only creature that as printed meets the criteria, but a half-celestial centaur, lamia, unicorn, or winter wolf can also easily meet the criteria. The giant bee and the spell phantom steed's effect almost meet the criteria. Details below.

Monster Manual Large creatures with land speeds of at least 30 ft. and fly speeds of at least 70 ft. with good or perfect maneuverability include...

Likely Choices

• The nightmare (MM 194-5) with fly 90 ft. (good). Explicitly allowed to become a cohort, the nightmare has a Level Adjustment of +4 making the typical nightmare ECL 10. Further, unlike many other creatures on this list, it's horse-like and, despite its Intelligence score of 13, allows itself to be ridden by, for example, the night hag (MM 193-4).

• An appropriate creature with the template half-celestial (MM 144-6) gains a fly speed equal to twice its base land speed and good maneuverability. (As opposed to the average maneuverability granted by the templates half-dragon (MM 146-7) and half fiend (MM 147-9).) The Level Adjustment for a creature with the template half-celestial is +4.

  Monster Manual Large creatures to which the half-celestial template can be applied with minimal fuss that then have a fly speed of at least 70 ft. include a blink dog (MM 28) advanced to at least 8 Hit Dice (minimum ECL 14), the centaur (MM 32) (ECL 10), an ethereal filcher (MM 104-5) advanced to at least 8 Hit Dice, an ethereal marauder (MM 105) advanced to at least 5 Hit Dice, an at least Large fire elemental (MM 98-99), a formian warrior (MM 109-10) advanced to at least 9 Hit Dice, a formian taskmaster (MM 109-10) advanced to at least 10 Hit Dice, the formian myrmarch (MM 110-1), the guardian naga (MM 192), a krenshar (MM 163) advanced to at least 5 Hit Dice (minimum ECL 11), the leonal (MM 142) advanced to at least 19 Hit Dice, the phase spider (MM 207-8), and the unicorn (MM 249-50) (ECL 12). Obviously, some of these make more sense than others. (A half-celestial centaur or unicorn? Sure. A half-celestial fire elemental or formian taskmaster? Not so much.)

  Because their alignments aren't usually compatible, creatures that are atypical prospects for the template half-celestial include the achaieri (MM 9-10), the annis (MM 143), the dark naga (MM 191-2), the displacer beast (MM 66-7) (ECL 14), the frost giant (MM 122-3) (ECL 22), a hellhound (MM 152-3) advanced to at least 9 Hit Dice (minimum ECL 16), the howler (MM 154-5) (ECL 13), the lamia (MM 165) (ECL 17), a shadow mastiff (MM 222) advanced to at least 7 Hit Dice (minimum ECL 18), the spirit naga (MM 192-3), the winter wolf (MM 256) (ECL 13), a worg (MM 256-7) advanced to at least 7 Hit Dice (minimum ECL 12), and a xill (MM 259-60) advanced to 9 Hit Dice (minimum ECL 17). Again, some of these make more sense than others. Also remember that even if a creature is listed as always being a certain alignment that always doesn't actually mean always (see Alignment on MM 305 and this question).

  What about half-celestial animals?

  The core rules don't really support animals being the targets of the 5th-level Drd spell awaken [trans] (PH 202) first then somehow gaining the normally inherited template half-celestial afterward, but if the DM can be convinced this happens somehow or that awakened once-animal-now-outsiders naturally roam wild somehow or whatever, animals that after gaining the template half-celestial that have fly speeds of at least 70 ft. include the bison (MM 269-70), brown bear (MM 269), camel (MM 270), dire bear (MM 63), dire boar (MM 63), dire lion (MM 63), dire tiger (MM 65), dire weasel (MM 65) advanced to at least 7 Hit Dice, a dire wolf (MM 65-6), heavy horse (MM 273), heavy warhorse (MM 273-4), hyena (MM 274) advanced to at least 4 Hit Dice, light horse (MM 273), light war horse (MM 274), polar bear (MM 269), lion (MM 274-5), tiger (MM 281-2), and wolf (MM 283) advanced to at least 4 Hit Dice.

  In all cases creatures without a listed ECL aren't given one here. The DM decides if a creature's ECL remains based on the creature's Hit Dice (like I've done here) when the creature's advanced to size Large; such a creature's Challenge Rating usually increase by an additional +1 when that happens (see Table 4–4: Improved Monster CR Increase on MM 294); what happens to its Level Adjustment is up to the DM.

  Thus the most likely and conservative choices are the ECL 10 half-celestial centaur with fly 100 ft. (good) and the ECL 12 half-celestial unicorn with fly 120 ft. (good). Easily adjudicated but less likely to occur in nature are the ECL 13 half-celestial winter wolf with fly 100 ft. (good) and the ECL 17 half-celestial lamia with fly 120 ft. (good).

Interesting and very close

• The giant bee (MM 284) advanced to at least 7 Hit Dice with fly 80 ft. (good). The giant bee's base land speed is only 20 ft., but listed here nonetheless because it's not a biped like the Unlikely Choices, below, and because the SRD says that a creature can take 6 months to make a Handle Animal skill check to rear vermin (DC 35 + vermin's Hit Dice) and 2 months to make a Handle Animal skill check to train vermin (DC 50 + the vermin's Hit Dice). Although listed in the SRD's epic section, such DCs can be achieved by non-epic characters, especially if the DM allows a bunch of folks to aid another already skilled trainer.
• If willing to settle for a slightly slower fly speed, creatures that can speak and wear a shoulders slot magic item should be able to make use of the wondrous item wings of flying (DMG 268) (58,000 gp; 2 lbs.) to gain a speed of fly 60 ft. (good). However, I figure the speed of 70 ft. is precisely to outpace those folks relying on such an item or an effect like the spell fly, so that's probably not an option.
• The 3rd-level Sor/Wiz spell phantom steed [conj] (PH 261) for 1 hour/level creates a quasireal horselike creature with a land speed of 20 ft./level (maximum 240 ft.). A level 12 caster's steed at will can use an effect like the 4th-level Clr spell air walk [trans] (PH 196) except that it lasts 1 round and that it takes no action to employ. This gives the steed an effective fly-by-running-through-the-air speed of 240 ft. and an effective maneuverability of perfect, but the air walk effect makes it subject to strong winds, the steed has only 7 +1/level hp, the rider plummets if the steed's dispelled or reduced to 0 hp, and such a steed just can't be a cohort.

Unlikely Choices

The DM may rule that these bipedal creatures aren't "suited for riding" so the rider suffers a −5 penalty on Ride skill checks when using one as a mount (PH 80). Otherwise, the following creatures meet the initial requirements, but their sheer power will likely make them unavailable as mounts.

• The astral deva (MM 11) advanced to at least 19 Hit Dice with fly 100 ft. (good). Minimum ECL 27.
• The avoral (MM 141-2) advanced to at least 15 Hit Dice with fly 90 ft. (good).
• The balor (MM 40-1) with fly 90 ft. (good).
• The brilani (MM 93-4) advanced to at least 13 Hit Dice with its fly 100 ft. (perfect). Minimum ECL 18.
• The ghaele (MM 94-5) advanced to 16 Hit Dice with its fly speed of 150 ft. (perfect).
• The planetar (MM 11-12) with fly 90 ft. (good).
• The solar (MM 12-13) with fly 150 ft. (good).
• The trumpet archon (MM 18-19) advanced to at least 19 Hit Dice with fly 100 ft. (good). Minimum ECL 27.

In all cases creatures without a listed Level Adjustment aren't given an ECL here. As listed under half-celestial, above, a creature that becomes Large has its CR increased by +1, and how this affects its LA is up to the DM.