You can get after-market rev counters. Traditionally, they just tap into the low voltage side of the
, but things have moved on since then, so I'm not sure how you'd go about wiring them. For the factory look, you'd need to get a replacement instrument cluster with the rev counter already in it. This might just work as a straight swap if you're lucky.
Swapping the rear drums for discs is likely to be a nightmare if the car has ABS, and if the car doesn't have ABS, then it's pretty much pointless anyway. Cars without ABS have a brake force limiter that prevents the rear wheels from locking up before the front (a front wheel skid is much easier to control than a rear wheel one.)
Discs are generally used on cars with ABS because of their added effectiveness (though some ABS equipped cars use rear drums.) The flip side is that drum brakes are better suited to being left on long-term (for handbrake use) and cars with rear discs have a tendency to suffer from siezed calipers.
Some designs have both discs for the driving brakes and small drums for the handbrake, though I don't think Renault ever used this particular idea.