Parts for e36 are consider cheap. Wear and tear stuffs are cheap like myvi parts. While body parts and interior  aere consider expensive as youre paying for halfcut/used stuffs.

For wear and tear parts, ill suggest get from Stuttgart at taman megah. Can google them.

I own an e36 325i and there is where i source my parts.

E36 is one of the outlier for conti maintenance cost. It is low cost and simple to work on.

M40 timing belt or m43 timing chain engine ?
Can’t comment on the m40. If it’s m43,
Check for perished ( on top of the usual things to check)
- Pcv valve rubber hose
- pcv valve
- plastic coolant flange on the rear of the block
- plastic coolant pipe under intake manifold

Check for operation
- disa valve if equipped

Ecu and tcu housing prone to water ingress. Make sure it’s dry and ecu’s have no corrosion

Parts for this engine is cheap. But it is a pretty rudimentary engine. 8v single cam. Quite a hardy little unit but makes sfa power. Not very difficult to work on.