Since anyone can buy a Droid with the prices in the rulebook, or use make up rules to build one, I have no problem just letting the PC's have an NPC droid in their crew. I'd just use the NPC stats.
As for adding things to droids, upgrading, etc., I think I'd use the weapon and vehicle modification rules as a basis, and go from there. Certainly it would require money and/or parts and a Mechanics roll (or computers for the brain.)
Adding something to a droid that could be purchased separately in the book anyway (say a scanner or something) isn't really going to unbalance the game, so just narratively make it part of the droid.
True enough. But with the remote I would effectively be turning it into a droid when before its basically treated as a piece of equipment. Granted the only thing I would want it to be able to do is slice computers, like R2D2 did. Thinking about it... it might be better if I simply narrated it and simply had it giving me a boost die?