1. I paint most of my models with a spray can. the thing is you have to use the correct technique to get it to look ok. if it's cool, warm the spray can by standing it in warm water. the heat will increase the pressure in the can and help the solvents mix. then shake can very well to mix up the paint. you need a primer that is compatible with your paint. spray it on thinly rather than thick. if it's not thick enough, make another coat after it's flashed (30 minutes). spray in short horizontal strokes and don't start or stop if you're aiming at your model (it might splatter when it starts or stops). Don't be afraid to practice on other stuff.
2. i don't sweat the 1/24~1/25 difference for dioramas. the rule is 'if it looks right, it IS right'
3. there are heaps of specialty shops that sell various aftermarket model parts. search for what you need. i'd leave adding more detail until you get the basics down. you can get diorama parts from a lot of online sellers and ebay (eg www.scalemodelingbychris.com). the doll section 'half-scale' will also get you some garage parts.