That looks perfectly suitable. One additional thing you should be aware of, though - if your existing plumbing contains any yellow brass fittings, they really need to come out because of a chemical quirk of PEX; the zinc gets leached out of yellow brass, leaving behind only soft copper, and the fittings may either leak or plug solid. You can use RED brass, just not YELLOW brass.
The manifold should be one size larger than the pipe supplying it.
If there's any question at all about grounding, a solid ground rod can be driven... with the result of giving you a better ground than you ever had by grounding through copper pipe.
Like copper, PEX requires a 1/4" drop per foot (never install it level or wavy), and requires a drain at every low point so the house can be winterized if necessary.
Is there any particular reason for going with PEX instead of tending to leaky fittings as you find them? I suspect that those fittings were always leaky because they were never sweated together correctly in the first place. I ask this question because replumbing the entire house in PEX is really a large, expensive job... while replacing (or resweating) one fitting at a time is relatively cheap.
I did this very thing (replaced copper with PEX for baseboard heating). But I did this because my copper had frozen and split in about 200 places.
So, if your copper is shot, I'd replace with PEX. But if the copper is fine, save the hassle!
But if you do decide to do it, some tips:
Sharkbite connectors are great and make it super easy to connect copper to PEX (at the baseboards)
Be sure to get oxygen barrier PEX. This is designed specifically for heating systems and has a metal liner inside to prevent air from permeating in.
pex is a pain for tight corners. You'll either need room for a gradual bend (they make elbow clamps for PEX to handle radiused corners) or you'll need to use elbow fittings for sharp corners (either those designed for the PEX you are using or, again, sharkbites--though sharkbites can get pricey)
Best Answer
Flow shouldn't be an issue. You can check the table here for copper and here for PEX to see how much pressure you'll lose per a given length of pipe. Copper and PEX are very similar.