Used parts - yes a lot of tikus stocks, curi case, etc. Prepare to bribe any visiting police officers who are looking for a quick buck or free upgrades to their vehicle.
Supply inconsistent yes too but can source from overseas though that depends on many other factors to consider.
Normal parts of wear and tear - tough business as you mentioned. Low margin, less popular brand, fast payment to supplier. In fact, many would want COD basis if your company is new unless you already built rapport with them but then the price would be 5-10% higher. Easy to test this - ask for quotation for 30/60 days term vs COD basis.
Yes, 300K very minimal nia. Considering the rent, deposit, equipment, labor, stocks. Tough business - high entry cost, low margin. Agree with those taukes. Eventually, you ll need to accept those tikus stocks, curi stocks, as those will yield margin.
From all the above it sounded very negative. So the question is why you would want to jump ship into it and put 300K or more into it plus dedicating 1 year of research?
For passion as an enthusiast?

As for Mudah, cost RM100 sell for RM300, nah, no more this kind of story. Mudah usually would be cheaper especially if more than 1 seller. If I m using Mudah and the stock is RM100 cost, I will sell for RM105-RM120 for advertisement purposes.

So far, yourself and me. 2 commenters out of 3 are of this line.

Determined, realistic and committed.
When you do start your business and if you re looking for suppliers, let me know. Will get a few for you at lower prices but better brands.

Parts you mentioned - timing belt, absorber, wiper, filter, spark plug, lub
Parts you did not mentioned also will have
But since now you re in research stage, I think it would be premature to introduce them to you at the moment. Whenever you re ready, just pm me will do. Then I ll intro them to you.
Have fun researching!