Hi and welcome to Sailnet.
I have a new to me 1985 Seafarer 26 with the 1gm diesel. I have 'some' stuff that came with the boat but it's mostly sales literature or user guides for the equipment the original owner put in. I'll look and see what all I have and let you know. There isn't a whole lot out there on the 'Net but some folks have already pointed you to what resources are available. I've found that most systems in boats are pretty much the same with very minor differences which can be worked by reading everything you can get your hands on that's even close. Sailnet is a great resource for that and you can usually find someone who's tackled that problem before and is willing to share.
You should be able to get any of the yanmar manuals, repair and parts, I did but they're not really cheap (about $40 each) but they do come in handy! Do a search on the 'Net for yanmar repair/parts manual. Right off the bat you'll want to replace the filters (oil and fuel) and check the air screen (mine doesn't have a filter, just a screen inside the housing) Yanmars want air and fuel but they want them separately! Change the oil and filter and change out the zinc. Other than that, I'd need more to go on than "it needs work".
I've found mine to be pretty responsive in all winds, though she seems to tip pretty good in stiff wind and I end up dumping a lot of air from the sails to keep her fairly flat. I bought it last spring and sailed it for the entire season without doing a whole lot to it but over the winter went through all of the major systems and repaired or replaced quite a lot.
cockpit drain hoses
fuel and oil filters
general clean and re-paint of engine
replaced plexi in both opening Bomar hatchs
replaced plexi in 4 opening portlights
teak oiled everything teak so many times I've lost count
The list continues . . . .
Good luck with your Seafarer! I think you'll like her.