It's probably worth it if you are inclined to do most of the restore work yourself, and are willing to be creative in sourcing parts. The trailer alone is probably worth the asking price. I paid $1900 for mine a few years ago, and have had a lot of fun restoring it.
Sure looks like a 240cc hard top to me. As you say, documentation is spotty on these old boats, so you would need to see if the manufacture placard is still in place to have a better idea of what you are really dealing with.
I took them off when I refinished. As others have said, those screws are probably due to be re-bedded anyway. That goes for all the teak on the boat, I found it a lot easier to work with in the shop.
Great looking boat! I'm with the Sikkins Cetol group as far as teak care goes. Cleaning and oiling looks great, but just doesn't last. As far as your shift issue, you might also want to look in to the shift-assist kit that Merc made for the engines of the era. They are still available.
The best answer I can give you as to value is to ask a question: how much work are you willing/able to do yourself? You are going to want to go thru the engines and outdrives, as well as assessing the condition of all systems, upgrading where necessary.
You probably have a bad steering cable. Not too difficult of a job to replace. Here is some basic information to help you get started: http://www.westmarine.com/webapp/wcs/stores/servlet/westadvisor/10001/-1/10001/MechanicalSteering.htm
I did the project you are contemplating last year. I used Interlux Briteside and Sikkens Cetol on the teak. I thought it was a nice update to my 78 Srv240.
In a 79 you probabaly have the pre-Alpha drive model 1R. That's what I have in mine. If you can get the serial off the drive I can confirm that for you.
I would recommend a rule full auto pump for the stepdown, I installed one in my '78 240, and it worked like a charm all season. As for the hatch, yes there should be a gasket in the groove that, when the hatch is dogged shut, makes it reasonably waterproof.