So a bit of background, I'm 16 and I have been regularly watching auto slalom and all kindes of racing that includes driving around cones. And I feel in love with Punto GT a long time ago, the problem is there is not a lot of these cars around.
I would get a Saxo VTS or a Pug 106 but there is not a lot of those too

So I found a few 1.6 mk1 Sportings and that's my last choice, I have experience around that engine (since my grandpa has swapped a 1.6 in his Yugo)

I will be looking for rust and I will check compression
Where should I look for rust?
Is 650eur a good price for 1.6 Sporting?
thx in advance

Something unique to the 1.6 is the position of the ecu directly beneath the heater matrix. When the heater matrix leaks (when, not if), it will damage the ecu.It is not just the water dripping on the ecu, but being contained behind the centre console, it creates a very humid environment. I have replaced the matrix on mine, but have also made a waterproof box to house the ecu, together with a sachet of silica gel just in case.

I will let others comment on bodywork issues as mine is a Cabrio.

How hard is to remove the ECU and heater matrix? What are the symptoms of a bad ECU?

sounds a good project [emoji1303], the general places for rust on the mk 1 are the floor areas/ sills also the rear inner arches to the boot floor as they can rust quite badly depending on what environment they have been used in .have a good poke around it may get you some money off it [emoji1303]

I'm so excited, for now the car will be registered on my dads name. I'm going to get eibach (-30mm) springs asap

The ecu will either work or it will not. If not, the engine won't run. Minor faults will throw up a code, but the problem is finding someone who can read it. The ecu and all the ignition components are GM units and unique to this engine. Removing the ecu requires pulling away the carpet on either side of the centre console. This will expose the ecu which sits in cradle which is bolted to the floor. To remove it, remove the two nuts on the front corners and loosen the nuts at the rear. Lift the front and slide it forwrad which will release the rear. You can then withdraw the whole unit to the side. The ecu is just push fitted to the cradle. When I waterproofed mine I removed the subframe bolted to the floor to provide sufficient clearance to enable me to fit a plastic lunch box which now seals and protects the ecu.
Replacing the heater matrix is a major job which requires removal of the entire dashboard. I did mine last winter. Replacing the actual matrix is the easy bit, it is just the amount of disassembly and assembly required for access which is the issue, that and discovering the bolts hidden behind soundproofing on the bulkhead in the engine bay!

Good advise already, rust will be your enemy, apart from the body & underneath, bolts nuts, brakes suspension parts & more, at an educated guess will be rusted tight. On a car of this age things may need changing. Penetrating oil & a source of heat will come in handy & be your friend....