l wanna understand diesels more than just book reading, so l figure rebuilding one would be the best approach other than taking some class.

'm looking for somewhere in the range of 70 is to mid 80's, definitely mechanical injection, manual tranny, and any 300D model other than the 300CD.
l know the injector pressure on the 300cd was slightly lower than the 300td or 300d is of the same time.l suspect the higher pressure injectors would be needed if l add a turbo, and the newer injectors are not interchangeable with the older ones.

any advice from someone who is rebuilt mercedes diesels?would this be a good car to learn from?

Pre 1986.After 85 they went to aluminum heads.You can ring them and sleeve them like most diesels.But must hone new sleeve in the block. l would have a machine shop do that part.

1985 on back 4 &5 cyl is in 123 bodies.
this being said & l do not wish to discourage u in any way. these engines require a few special tools & a few special procedures the parts r expensive & the service procedures must be followed EXACTLY & in exact order.
for a successful outcome u cannot cheap out any aspect of this engine & when u have a completed long block that is in perfect order u still may have to contend with a worn injection pump/governor [if u send the injection pump to a pump shop [for an estimate] first [before engine tear-down] u will have more knowledge of what the complete overhaul costs involved for this engine will be.
now with all the above being said l would not advise this engine to ''learn on'' this is not to say that an amateur can not do the job properly l have seen ''ams'' with knowledge & care achieve perfect results on this engine. but mostly l have seen a lot of expensively destroyed parts due to uncleanliness CHEMICALLY CLEAN PARTS TO BE ASSEMBLED IN A STERILE ENVIRONMENT, no care[WRONG TOOLS], stupidity[ BUY A SHOP MANUAL AND FOLLOW IT] & the big issue of low bucking DON'T CHEAP OUT THE JOB.

l have an '81 240D (4 cylinder version of the 300D, manual trans, no turbo) that was my first Diesel, & having owned Audi, BMW, & a bunch of American cars, this car is a dream to work on, a pleasure to drive, & very cheap to run & maintain. l bought it in part to learn about how Diesels work, & the first few times l opened the hood l just stared at the engine like an idiot. but now l am really comfortable with it, & can diagnose most problems just by listening.

l have not rebuilt the engine, but I've done a lot of work on it, including replacing the timing chain & removing the injector pump (quick & easy on the 240D, a lot of extra work on the 300D due to the relative lack of space). A previous owner had the cam timing off by one tooth, & the injection pump off by a tooth in the opposite direction, though the car still ran okay. l would highly recommend going with an '81 or newer, because of improvements to the cylinder head, with a better glow plug circuit - pencil type run in parallel rather than the series kind where if u lose one u lose them all. One word of warning, though, NEVER use Autolite; use Bosch - they can be had online for $9 so there is no reason not to.

You do need to make sure u have injectors with the correct pressure rating, but Bosch sells remanufactured ones that work great & r not too expensive. l built an injector tester with a grease gun & some parts l had lying around - they should be well matched for the engine to run smoothly.

l realize l have not really answered ur question, but it is neat to see anyone getting into these cars; if u have a rational mind they're very rewarding (unlike working on a Ford). l would imagine rebuilding would involve extra critical levels of precision, since these cars were meant to be driven for 300,000 miles without a problem, assuming proper maintenance. The mechanical injection pump is something I'd be afraid to get into without a degree in mechanical engineering, but with a good manual & a spare one to use as a guinea pig, anything is possible. Good luck, & enjoy!