It will still crank with crank sensor failed.
Battery needs load testing properly. If that's ok, the battery fuses. If they're ok, then start looking through the power cables and earths.
Thicker oil won’t be helping, as @burrellbloke can attest to.
Stick with the standard grade.
The weak link is always going exist on these. Just treat it as a wear item as you would anything else.
Whatever way you look at it, generally speaking, putting a Megane engine in it will cost at least double what it would putting another 197 engine back in it.
They’re slightly different. 197 is 830 engine code, 200 is 832.
The 200 has tweaked cam geometry, but essentially there’s no real change.
The difference in gearbox between the 197 and 200 is more noticeable.