I would go with either your first or third option.  I have not looked at the kit that Aerostitch sells so I could be wrong but, there is a good possibility your will be getting certain tools you won't need.

Most of the newer non airhead BMW bikes switched from Allen style bolts to Torx (if that is the correct term) bolts.  So many new kits will have a selection of Torx bits or sockets that you won't ever need and you may not get all the Allen stuff you do need.  You may not get the box/combo wrench that fits the bigger steering head nuts as well as the tool to adjust the bearings.  Mind you, I don't have my stock tool kit in front of me so perhaps there are other BMW tools in the Airhead OM kit that isn't in the generic Aerostich kit.

I have no idea what stock tool kits are selling for on the market.  I would assume them to be reasonably priced.  The quality for the kit's tools is pretty good, lol far better than the Suzuki kit my cousin's bike came with.   In a pinch, just about any task can be done on an airhead with the stock kit.

As for building a kit from scratch, it can be done, not sure of the cost.

Best world is to have the stock kit and add to it.  In my case, I added some extra wrenches, better pliers, just to name two items.

Good luck, St.

I recommend OEM tool kits for airheads. I got a very nice new one of quality tools from Bob's BMW for my R65 LS and I'm sure they have ones for the R100 RS. 

You can easily put your own together but it's a bit of messing around figuring out just what you need. 

Complete, OEM tool kits are on eBay all the time. 

R65 kit

Thanks for the advice. I went ahead and ordered an OEM tool kit from Max BMW.

It should keep you in good stead. ou might want to add a 3rd tire iron to your kit depending how good you are at changing flats.