As far as reliability goes, the 18rc is as bullet proof as the rest of the R engine series. If you are looking for performance out of the engine, I'd say just do a swap with an 18rg or 20/22R hybrid and be done with it. The RC is the single cam economy version of the RG, so you won't get a whole lot out of it without some substantial work done. If you just want a bit more power you could always do a pair of sidedrafts and bigger exhaust and you'd probably be happy with that. As far as its "exclusivity" goes, it isn't. These came in all sorts of small Toyotas from the era including all US Celicas 73-75, and I'm sure Coronas etc. It isn't going to be worth anything to anybody, unless it is a running swap for someone who just needs a working engine. All that being said, I drive the holy hell out of my 73 celica with stock 18rc and about 200k on the clock. It is enough fun to keep me enjoying the car, but not enough to get me into trouble on the road... Of course I do have a complete running RG swap waiting in my garage...
I was originally under the impression that the RC was only offered in California, and therefore had some scarcity behind it...but I guess I was mistaken! Well then I guess I'm stuck with the RC for now!!
Because I'm not really wanting to put out a few grand for the RG (unless theres somewhere to buy them for cheap?), and because I don't really have the time to do the whole 20/22R hybrid swap (unless they're actually pretty easy?)