Good call mate

ha!
Yes, as
@Ricardo mentions the easiest way is by using the Ktec custom seat bases, then you'd need side mounts for the seat as well (generally a universal item around £30), and also a slider unless you are happy for the bucket seat to be fixed in the 1 position.
Here's the subframes:
K-Tec Racing - Renault Tuning Styling Motorsport Specialists
They do lower the seating position from the standard recaros, but its not as low as some would like - I would ideally prefer it 1 inch lower, but I am 1.94m. These bases are a direct fit to the actual floor of the megane, but for a wide(ish) seat (again I'm 92kg or so and went for the 'XL' width seats) you need to have the ktec bases drilled out in order to get the side mounts in the correct position. I had Rentec in Edinburgh do mine, if you are handy with a drill, it should be very easy.
Seat belts: you theoretically could, but it would be a lot of custom work I'd think, the biggest issue, is that the seatbelt clip is mounted to the recaro base, and the pre tensioner relies on that as well etc, I'd be worried about the belt not functioning as designed with a bucket seat - I switched to harnesses.
Ideally, if you can, I'd stick the buckets and a £500 rear cage in at the same time. Fixed back bucket seats have ZERO give in them in the event of a nasty incident, where a normal reclining seat will collapse and not crush your body in a similar situation, the fixed back keeps your neck in the same place.
Ricardo's thread has really good pictures on how to fit the buckets, -
Ricardo's Budget Track 225
and my one will give you a good indication as well mate.
R26 - Standard to Track
1 thing I would say, is make sure you sit in the buckets first!, I had an expensive mistake when I bought slightly too tight seats!
Also worth noting, you should be able to get £600 for your old seats if you are selling them - they help out with the costs a lot!
let us know if you have any more questions?