[WillyF]

I've heard that Google will sometimes "reset" link value for domains that change ownership; however, I don't know of any specific examples of this happening. There are plenty of sites that get sold for their SEO value and seem to avoid this problem. SEO traffic can disappear at any moment—especially if you're not a major brand—so I'd be nervous about making an investment that takes 3 years to recoup.

I'm not sure if it's the right move to buy, but if you do I'd keep the site stable for at least a month or two. Then start making changes gradually. You don't want to end up in a situation where you make a dozen changes and your rankings tank. You won't know what caused it.

You absolutely need to maintain your URL structure, or use 301 redirects to maintain your link equity.

The "dinosaur content" is what's bringing in the traffic. Don't ever get rid of it, even if you end up deemphasizing it long term.

My take (from your questions) is that you're not ready to make a purchase like this. You should start working on your own project where mistakes won't cost you tens of thousands of dollars. Once you get a hang of things, then you can start buying sites.

One site that I love is http://selfmademinds.com/

It's the blog of two guys who buy/sell/lease/flip domains and sites. Lots to learn from them.