A power trencher, or Ditch Witch, is a fairly easy machine to use. Most are self propelled. You may check some local rental stores as well as HD, as you will probably need a trailer to transport the machine and most rental stores include it in the price.
Before you attempt to use one of these monsters, be absolutely sure to call DIG SAFE @ 1800-digsafe. You must have the utility companies come and certify that there are no buried lines, wires, gas etc. It is the law, so don't forget to get it done. There is no charge for this service. If you don't and hit something, you can be liable for huge costs and fines.
A Ditch Witch usually only digs a narrow path for laying wire or pipe. The path width would be a bit shy for a good drainage system as you will not have enough width to add the proper crushed stone base and surrounding gravel bed etc.
Assuming you are installing wire or pipe, be sure to get some warning tape to install a few inches below the ground grade to warn future excavations that utilities are buried benieth. Good Luck.
I think you're on the right track, especially in considering TWO saws, one high-speed saw dedicated to wood and another very different low-speed saw for brass/steel. The wood saw will be fine (but messy) with alumin(i)um.
You should know a couple more more points.
First, the kerf on nearly all bandsaws (except sawmill types) is pretty much identical regardless of blade choice.
Second, a saw with a gearbox (don't DO this with a variable-speed motor!) for two very, very different speeds (wood and alumin(i)um require MUCH higher speed than steel will tolerate) will tend to be a very high-ticket item, generally much more expensive than two dedicated saws because of the dramatic shiftable gear reduction required for steel.
Now... to really answer your question of must-haves, we should know what - in particular - you plan to cut, and how accurately you want to cut it. What thicknesses of wood and metals? Do you ever plan to cut steel or brass (if not, you could do with one saw)? Do you require high-quality cuts in non-alumin(i)um metals... or could you do that work with a reciprocating saw? In wood, what are your goals? How large (length / breadth) a workpiece do you really plan to cut curves in?
For most steel I imagine you cutting, A small (4") vertical or vertical-horizontal bandsaw would suffice. I have one such from Harbor Freight in my shop, and it does everything I want a steel-sawing saw to do (which is really quite a lot).
A 14" bandsaw is the de facto standard for a "serious" woodworking shop. A wide array of manufacturers offer them, they all take one common blade size (unless they have a "riser" to lift the upper wheel another 6", in which case they all take a second common blade size).
You should know... that most bandsaws have a considerably smaller vertical capacity than the saw's "inch" size (which is a diametric measurement of the upper and lower wheels). Most saws have about a 6" vertical capacity, and that's really pretty realistic unless the saw is equipped with a very large motor; cutting material more than 6" thick requires a GREAT deal of power. A riser will gain more height capacity, but it's nearly always wasted capacity because the motor can't manage resawing stock that's much more than 6" thick (unless it happens to be balsa or basswood or tupelo or other very soft wood).
I like the 14" saws; they're generally built much sturdier than smaller saws, and have better blade guides. Blades are easy to come by. The basic saw, with no "upgrades" (some of which are glitz with no real practical upgrade value at all), works well for most tasks. Because they're so very common, there's lots of help available on the Internet if you should run into trouble aligning, maintaining, or using the saw.
I apologize in advance - I'm afraid I've rambled a bit here...
Best Answer
It's physically very difficult to dig a very narrow but deep trench. You may have better luck just renting a ditch digger/trencher, which is kind of like a big chainsaw for dirt. You can dig trenches that are very narrow, and the only thing that really slows you down with it is large rocks.
You should have utilities marked before you dig (whether using a trencher or not). In some areas, this is enforced by law (eg: in Ontario, Canada, it is law under the Ontario Occupational Health And Safety Act).