Hate to say it, but putting Volvo and cheap in the same sentence doesn't work.
You could probably have your existing pump rebuilt for what you'd spend for another used (and probably bad) one. Same for the injectors.
I'm not familiar with the MD-7 engine, but if it's anything like the TAMD-120, you'll need to buy or make a special injector extractor tool. If you have the shop manual for the engine, you can find out. BTW, having the shop manual is well worth the cost of the book.
This is a link to the do-it-yourself manual directly from Volvo:
Do it yourself books : Volvo Penta
This is a link to a third party company for the main shop manual:
Volvo Penta Auxiliary Marine Diesel Engine Manuals
They're worth the money if you're going to troubleshoot and repair yourself.