The vsr-10 has the most aftermarket parts as far as I know. Also, the hopup is superior to the others. After all, it is a marui.
$1000 will get you virtually all the upgrades. The biggest "blocks" will be a new trigger mech (depending on how hot you want to run it) and the barrel. The rest are smaller purchases, probably around 30-50$. Also, you'll probably want a new cylinder in addition to the other performance upgrades. It makes life easy. Finally, I believe the consensus is that PDI performs better than laylax upgrades, but I went the laylax route and am very happy.