I started a separate thread around general technological expectations of the Star Wars Universe stemming out of the discussions in this thread.
In terms of Droids, remember that PC droids are different from "stock" droids as well, in that their sentience is on a different level than from most others that one might encounter. They actively seek to improve themselves beyond their basic programming. As such, it makes sense for them to have the same limitations that organic PCs. It isn't about min-maxing at all. That having been said, Droid characters come with more experience than any other race, and they get to pick six career and three specialization skills right off the hop. They already have a fair bit built in. Purchasing upgrades is the same for any other droid.
C-3P0 has an interesting history as a "PC" droid. Episode I has him built by Anakin Skywalker. While this is partially true, he is still a mass produced model of the 3P0 series of droids. He was first activated in 112 BBY (I think)... Serving in different capacities before running afoul of something on Tatooine, where he was effectively junked. Anakin found him and started rebuilding him, and he got a new lease on life. Of course, his memory would have been wiped more than once in this capacity.
In terms of droids with more built-in features, the probe droids have built in weaponry, hyperwave communications capability, repulsorlift transport capacity, and of course, lots of sensors. There's precedence for it, but one cannot expect a droid like that right out of the box. R2-D2 was a veritable Swiss Army knife, but that could be simulated in two different ways: one is to have him "equipped" with a utility belt representing a number of his apparent features. Secondly, it seemed that he might have had the "utility belt" talent, being able to produce a particular bit of gear (or in his case, yet to be known functionality) as the plot required. Certainly R2 was not armed with anything lethal. His little stun wand would have been another piece of equipment that he "learned" how to convert into a very short range equivalent of shock gloves... Which would have been represented by paying the cost of shock gloves, and it manifests in the way it did in the films.
I think having this talk about the true utility of droids is helpful, but also to set expectations at the point where PC droids are actively seeking to go beyond their base manufacturing specs. Even so, looking up Droid examples in the adversaries section can at least give them an idea of where to start. Equipment and xp = upgrades beyond basic manufacturing specs in most cases.