Currently, I have a group that owns a surgical droid that is able to be upgraded, however, I have not yet decided on the mechanic for them to be able to do this.

Originally, I said that it can gather XP with the players. When spent, though, all skills are 5x rank +5 XP, and characteristics are always 10x rank +10 XP. I did this to create a sink so that the droid did not become too powerful, but I'm having second thoughts on my system.

I'd really like to be able to have them upgrade it based on a Mechanics check, so it deals more with the players' skills. Does anyone else have a system in place for upgrading something like this?

Side question: Should I allow them at all to be able to upgrade any combat skills for this droid? One player wants an army droids at his side, but I'm having a hard enough time as it is trying to create a challenge for this group.

Do you have the EotE GM Kit? The advancement rules for Nemeses are perfect for this.

Integrated medical equipment at cost listed in book plus a mechanics check.

Same with an integrated blaster of some kind. I assume the droid doesn't have any skill upgrades, so maybe let them buy some downloadable software. 1000 credits to give him one rank in one combat skill seems fair, along with a computers check of course.

He's probably only got an agility of two at most. So YG for ranged checks with the above. Not particularly terrifying.

Thanks for the help, both of you!

Also, regarding the "army of droids" concept, the AoR GM Kit can handle this in a pretty darn well balanced way with the squad rules that, and liberal use of the "we don't serve their kind here" trope. "Your droids—they'll have to wait outside."

Also, the potential on a Despair (or using a dark Destiny Point) to have a droid malfunction. If they start having their droids in combat, maybe one of them has their friend-or-foe subroutine accidently switch the two and it starts attacking the PCs. If you've run them through the EOTE GM Kit adventure, the possibility of their droids getting infected software becomes a lot of fun.

Short version: be creative with the unique advantages and challenges of (semi-)sentient electronic henchmen, and have fun!

Frankly, I suggest a bit less transparency about the droid's advancement and capabilities. Give the PCs a choice - regular wipes or no regular wipes. If they do the wipes, the droid doesn't gain experience - but they keep complete control. If they don't, you get to treat it like an NPC, with personality quirks and motivations. But the droid gains experience - even if they can't exactly control in what area.