Eric's right; there's any number of ways to do it. But by way of starting to decide, you may want to ask yourself to what degree you're going to carry your paint job. If you're simply going to spray the model green and quit, then by all means assemble everything and then paint, but if you are considering a good wash and drybrush, you might want to consider painting and weathering in sections. Proving Eric's point again, I like to paint the hull, turret, road wheels, track, and occasionally the pioneer tools and other add-ons as separate entities. Once I've washed and weathered, then I'll assemble them all. Doing it this way allows for better access, a better means of holding the parts ( I can stick several fingers through the hull and hold it), and a little better detailing advantage. But that's just me. Try it a couple different ways and see what you like and what works for you.
Gip Winecoff