The short answer to your question, as mentioned before, is that there are no kits. I think the fact that lucas frowns on that is only a secondary reason for this. The main reason is that builders needs and desires are so diverse that it would be really difficult to make a "one size fits all" kit that appealed to everyone.
In general, everyone comes at an R2 with a different budget, a different toolset, and different areas of expertise and familiaritiy. Some people know how to machine but couldn't glue styrene together to save their lives, and some folks are the exact opposite. And there's everywhere in between.
The advice I always give to people considering building an R2 is this: first, decide what you want your R2 to do. Second, figure out your budget. Third, calculate which parts of your requirements can be completed with your budget.
I find that typically budget dictates materials. If you are going for an inexpensive build (relatively speaking) then you'll want to focus on styrene, wood, and resin. If you have some extra money, you can upgrade some parts to aluminum, and work in some extra gadgets.
My one piece of free advice that I will stick to until I die is this: there's ZERO reason or benefit to making an all metal droid. Unless you consider saying "I have an all metal droid" reason to do it. There's just no advantage. Anyone that tells you that they are more structurally sound and robust are selling you a bill of good. To be precise, it is true that an all metal droid may be more structurally sound. HOWEVER, there are NO circumstances in the natural world where you will need a droid that rugged. in fact ,there's a counter argument to be made that the slight flexibility offered by a wood frame is advantageous, as it can manage stresses better than a metal one.
As you dig into the R2 world, you'll find that there are a lot of "religious" debates like that one going around that have no right or wrong answer, but is really more geared toward your budgted, skills and toolset. For my money, I would never build an all metal droid again. My second R2 currently in progress is wood, resin, and a bit of metal. the ONLY things you need to have metal on your droid are the dome and the skins. If you're making a "show" droid, upgrade parts to metal that LOOK LIKE METAL on the real guy. If a part is painted blue, use resin.
