Maxim/Vickers owner here, but I neither own an MG08 nor an MG08/15.
Maxims are generally durable guns, but the main spares you will want are firing pins and lock springs. Both should be adaptable from Russian 1910 parts that are more commonly available, but drying up as we speak.
The original German locks do not allow headspace adjustments as noted. The 54R conversion replaces the internals with Russian parts and gets you a lock that can be headspaced. The conversion does not require a complete Russian kit, but the parts are notoriously difficult to locate separately. Kits will run from $2,000 to $2,500 right now.
Black River Militaria can do the conversion mods, but I can't remember what Bob charges. You'd easily spend another $3,000+ to get the conversion going, which would buy a ton of 8mm reloading components.
There are techniques to keep the gun running when the headspace starts to run long, but we can save that for later.
I'd love to have an MG08, but many are in rough shape or missing parts. The conversion aspect keeps me away as well