Solved

Car condition affect anything?

50% of what? engine? If you do mean engine then yes 50% engine capacity will run operate at 50%. Wouldn't recommend normally to let the engine wear down to that level. Normally an engine should be used for 2 races before you change them but if you have enough financial stability and are challenging for championship then changing engines each race could be the way to go. As every bit of engine power will be crucial to get ahead of the competition. So using an engine running at 50% compared to a competitor running new engine you won't stand a chance.

Repairing the car is also very important using parts because if the car isn't repaired but had new engine you won't get full power and it will show you down so to get max performance repair car using parts each race but change engine every 2 races unless you need more performance change engine every race as well. But if you are struggling financially then you could stretch each engine to maybe 3 races. That's for you to decide and manage. For a 2 car team you recieve 20 engines which are RESTOCKED every 20 races if you use them up or not every 20 races you will get RESTOCKED to 20 engines. If you purchase additional engines and don't use them all up on restock date you will lose the additional engines and engine stock max will be 20. You cannot stock them up more than 20 to keep it fair and competitive for all teams.

Also read up on these guides they explain stuff in more detail. 
https://igpmanager.com/forum-thread/8050

https://igpmanager.com/forum-thread/8281