Solved

manufacturing building

If you don't upgrade it you will run out of parts to repair your cars.

The consequences of your decision will become evident as you level up and continue to develop your design building (developing the Design HQ will increase the wear rate of the cars).

Unlike engines, parts usage is cumulative so you can't make savings by only repairing (for example) every third race. When you do repair after the third race you will simply use three times as many parts. If you choose not to repair your cars for too many races they will suffer suspension failure and DNF.

The only effective parts saving tactic I have found is to not repair cars for the final two or three races of the season because the cars will be repaired for free during the end of season reset process.

You are in a one-car league which helps because the number of parts the manufacturing facility generates is the same whether you're in a one or two car league but you only need to repair one car.  

In conclusion, if you choose to keep your Manufacturing facility at L2 you may eventually regret this decision because you will need to buy parts with tokens to prevent the cars breaking down.

My simple advice has always been to upgrade Design and Manufacturing HQs as a pair, keeping them as close as  possible to the same level. It's not quite this simple because as stated earlier it's easier to balance parts in a one car league.