With 300K miles on your 240D it is a tough call. You can spend $1.5-2k for a head gasket and head but the bottom end may be getting weak (you really need to fix the vacuum pump either way). You can find a decent used 616 and install it, maybe $3000 if you pay to have the work done (but it's still a risk buying a used engine). Finally, you can buy a reman engine and have it installed which is likely to cost $7K by the time you're done.

My current approach is to find a low mileage used engine and try to make sure it's really good before it's installed (I plan on having my indy do a leak test, replace the major gaskets, check the chain, adjust valves, time the IP, etc.). I also need to replace my oil cooler and hoses. Even with a used engine I expect to pay about $4K; a reman would probably end up costing closer to $8K.