For 150k euros you can build decent R4 I think. For such price you can't get any good R5, but that is just as an information.
Running costs should be some 30-40% less than R5. Spare parts for sure cheaper, as most parts you can buy from external suppliers, not directly from Oreca. Engine with auxilliaries is almost standard, so cheaper to run and rebuild, transmission possible that little cheaper also. Suspension parts from kit not more expensive, other parts not from kit sure also much cheaper. Running cost/performance ratio attractive for sure, buying/building cost is only challenge...
Car is really nice, on tarmac like gokart. Gravel also good, as suspension stroke is longer than R5. Engine quite good, less advanced than R5 but 33mm restrictor helps, plus good mapping, good antilag, hopefully better management of pop-off than most R5...
In my opinion if you spend little bit less than new r5 then you can have similar fast car, need money for good parts plus good engineering.
If car build budget way then decent machinery still and cost of building/buying little smaller plus running cost significantly lower.
It good opportunity if you can have some financial support of local/national dealer, etc.