airbrush is probably the best tool for that job, but you are right, they have a steep learning curve - and decent ones are pricey.
Thin paints and a good 1/4\" or 1/2\" flat brush red sable or better should do you a decent job (go to an art store Michaels, Pearls, etc. expect to spend a least $10 on the brush not one of the five for a dollar packs)...Spray prime. Base coat. If you cover your primer in one coat, you are not thinning your paints enough. Two to 3 layers is not unusual. Let dry between coats until you build up the base color. Then go back with even thinner paints and do a wash in the recesses where the paints would be darker. Follow with a lighter layer in the areas that would be sunbleached or would be worn.
Details can make the tank look very sci-fi or very battle weary... muddy tracks and undercarriage, rusty bits or grease runs at hinges, muddy tracks where the crew climbs in and out, etc. These all make the tank look more believable.