Hey Roland,

A lot of comments supporting the use of drill motors instead, and I agree that for a first robot, they are a very good and proven way to go. However Speed 900s are also very good motors and if money isn't a problem and you can go for a decent set of machined gearboxes and transmission, then they will also serve you well.

You talk about having four motors and connecting all the wheels so that if one motor fails the other one can still drive all the wheels. In my opinion, that's a bit overkill. Yes it provides a level of back-up/redundancy and therefore improves reliability, but it's a bit of a waste of weight in a featherweight. Speed 900s weigh roughly 650g so 1.35kg for two, plus extra weight in gears and gearbox housings (could be almost 20% of the max weight); it's a lot of unnecessary weight that could be used to beef up the chassis or armour of your robot. Speed 900s are also incredibly reliable motors from my experience so there shouldn't be much to worry about in terms of motor failure (famous last words though )

If you go for them, I'd recommend a Speed 900/gearbox combo directly driving one wheel on each side, and then slave the other four wheels with timing belts and pulleys. A drill motor pusher is a perfectly fine robot, but you will get a much better, faster, more powerful performance from the Speed 900 arrangement.

I'm a perfectionist, I hate doing things and finding out they don't work
I consider myself a bit of a perfectionist too, but in this hobby, you'll find yourself doing a lot of things that end up not working. Learning from them and improving upon them is part of the fun and part of the challenge