No matter what the cost, I'd definitely go for:
1) the new 2.0 16V turbo!

Why?
- just to be the top dog in the R9/11 society!

However, the new engine with all its ancillaries would out-value my 9 by at least 60 times!

So I guess I'd have to go mental with the rest of the car, too. But, money no object, that wouldn't be a problem...
But, keeping things realistic, if cost is a matter of consideration I'd probably choose:
3) Volvo 1.7 turbo. Why?
- turbo-charged, so more power is easily achieved by upping the boost slightly. Standard maximum boost is about 0.5 bar (which is very low), so upping it to just 0.7-0.9 bar would probably bring the power already to the 140-170 bhp region without too much other modifications needed, without compromising reliability too much.
- multi-point fuel-injected with knock sensor & ignition retarding, so it self-adjusts (to a point) if the boost is upped a little. So no need to e.g. re-jet like you'd have to if it was a 1.4 carb turbo.
- relatively easy to source a donor car (in Europe)
- valve adjustments not needed as nearly as often as in the push-rod 1.4
- oil-sprays cool the pistons from below, a definite plus over the 1.4 turbo
- bigger displacement means better torque over 1.4T
- it's a different solution: I haven't seen too many R9/Alliance 1.7 Turbos...
The 1.8/2.0 16V is a great engine, but tuning an atmospheric engine is usually more expensive than (mildly) tuning a turbo engine. This is because for an atmo you need to have parts machined (like the head flowed, cams reprofiled), whereas for a turbo upping the boost is almost all that is needed.
The new 2.0 16V turbo (code F4R-762 when coupled with a 6-sp. manual, F4R-767 with a 5-sp. auto 'box) is based on the same old trusty F-series engine as the R9/11 1721 cm³, so therefore it should physically bolt in place. Provided they haven't changed the block castings. These new engines are all about electronics, so a carburettor would probably not be a great idea...
What ever the engine, be sure to fit an engine oil-cooler! Oil cooler wasn't standard on all R19 16Vs, and quite a few suffered a disaster as hot oil started bubbling in the circulation, causing oil starvation, causing a hole in the cylinder block!
AFAIK, no Renault using the JB/JC series gearbox has had a limited slip diff in series production. Renault Sport may have offered a kit when these 'boxes were still used in competition, but I assume today parts are non-existing or very pricey at best.
I paid 120 euros for the rear axle (with torsion bars, rear hubs/discs and pressure-regulating valve components,
sans brake calipers/cylinders)