You can get all the parts, it just takes a little longer because you cant get them from the MB dealer or your local auto parts store. You have to get them from places like Euro Truck, Peach Parts, fourbyfourclub dot com, or simply order direct from Germany etc. Some of the parts will not be in stock anywhere and will have to come from Germany. It took about 6 weeks for me to get the brake parts for the rear on my 96. Just keep in mind the parts are very expensive unless any parts are shared with the cars. The rear brakes cost me around $600 (parts only) but the front rotors only $75 off ebay for the pair because they are also on the big sedans. Often times most of the engine components will be on the cars, but it can be tricky to get them because you have to call it by the the car name rather than the G, and then its not guaranteed to be the same part....so you will need to do lots of research, checking forums etc for specific parts and DIY projects.
Basically the cheaper you go the more elbow grease you need to put into it. But that can be rewarding if you like to work on cars. Once you get all deferred maintenance issue out of the way and all the fluids replaced you will have a good and very reliable truck. The 300 is good, the bigger the engine the better. All the older ones are a bit under powered compared to the new ones, but it just takes a little longer to get there, not a big deal.