There is a lot of good advice here, and as a long-time owner who is a retired nuclear engineer who can do just about anything on my NSX, I would be reluctant to buy this car myself. And that is knowing that everything that is needed to be done to get this car back on the road is probably within my capability (minus evacuating the Air Conditioning system) on jack stands in my garage.
And, yes, evacuating the A/C system will need to be done since the refrigerant charge (and this is a US 1991 NSX, so the default is R-12 (freon) for the refrigerant) is something that you need to get right in terms of the proper number of grams of refrigerant that should be in the system. And that is just to start troubleshooting/verifying the system is operational.
Since you can do basics like oil changes and spark plugs, but not much beyond, labor costs are a big issue. I would budget 100 Euros/hour (and that is a pretty good labor rate). Just the Timing Belt/Water Pump (and if the car hasn't really ran in the last 10 years, it's due) will run you between 1000-2000 Euros - depending on what else you do at that time (timing belt, water pump, plugs, valve adjustment, coolant flush and fill, fuel filter, alternator belt, A/C belt, transmission oil change, engine oil change, brake fluid flush, coolant hoses, evacuating and recharging the A/C will also be likely just to start troubleshooting the A/C system) - in labor alone. And then you gotta get the parts. Sadly, some pretty important parts are discontinued from Honda. But as a reference on parts costs, I'm already starting to build up my parts bin for my next Timing Belt/Water Pump service isn't due for another 3-4 years. I just got a new water pump, timing belt, timing belt tensioner pulley, timing belt tensioner spring, and lower timing belt cover gasket for 635 Euros...and that is EU export (no VAT/other taxes) and before the 4/1 price increase. And that doesn't include parts like the coolant itself (expect to pay 50-100 Euros just for the coolant alone), the oil filter/cooler pedestal gasket, the special bolts for the oil filter/cooler pedestal, special bolts for the water pump, valve cover gaskets (another 210 Euros right now), various o-rings, grommets, crush washers, filters, and oil.
And if you do the coolant hoses (since I'm willing to bet a beer that the hoses are 29-30 years old)...that will run you - depending on where you get the hoses - several hundred euros in parts. And the labor time for the hoses will probably land somewhere in the 10-15 hours range..."book time" in the US is 14.5 hours to do all 22 coolant hoses on a 1991 MT NSX...an experienced NSX mechanic may quote a little less, but still expect 1000 Euros in labor minimum for the coolant hoses
Just to get that car on the road safely, I would budget between 5,000 and 10,000 Euros parts and labor. That's on top of the 30,000 Euros or so that the owner is asking.
There just seems like so much that will need to be done just to get the car to even start to become a car you can just hop in and take for a drive without any concerns for reliability.
Now, if you do decide to pull the trigger and buy this car, we will be here. The community is very helpful when it comes to finding rare/discontinued parts as well as providing maintenance guidance. But I also think you'll find out the saying, "The most expensive NSX I ever bought was a cheap NSX"