I probably wouldn't mess with paint unless I absolutely needed to for detail... I've found a few that say they are aquarium safe, but even then they'd have to be coated with epoxy or algae eaters would munch it right off They seem to enjoy that gold paint that Penn Plax uses on some of it's ornaments, and I've got some Aquatic Gardens ruins that they've almost stripped to the gray plastic.
I don't think aquarium sealant would be too difficult to work with... I seem to be using a lot of clear silicone latetly (not aquarium related though) and seem to be getting pretty good with the stuff... 2 part epoxy is the other option, but I've had a lot of problems with epoxy not fully curing in the past.
I just like the idea... With scale models you get much more detail, and you usually have size choices, and can find just about any model you are looking for to match the size of the tank. Maybe even the fuselage of a big b-52 model or something like that would be cool with fish swimming around in it... Just break the wings off, cut it in half, and pop some nice holes in it. This gives me all kinds of cool decor ideas.