Welcome to UKY from a fellow New Englander (grew up in CT, moved to MA for school and employment)!
1) My job prospects -- legally, I suppose I will be "able" to work, but will anyone want to hire me?
Are you wondering that UK employers might be hesitant about employing an American?
I personally haven't experienced any bias in my job search for being an American. In my experience, it's just like a job search in America - they evaluate your previous work experience, how much you know about the field/industry, and during an interview, what you're like as a person (ex. friendly, aloof, engaging, arrogant, etc). Being an American has, in my experience, had nothing to do with my being hired. It was because I had the most previous experience for the role than the other applicants.
2) Is there anything different about applying for/getting a visa if we are both moving back at the same time? Does it really cost 750 GBP to get a settlement visa?
The visa process will be the same regardless if you move over separately or move over at the same time.
The current cost of a spouse visa is £750.
http://www.ukvisas.gov.uk/en/howtoapply/visafees/visafeessettlementThe Consular exchange rate is currently $1.65 to £1, so the cost of a spouse visa in USD is 1238.
Visas can take several months to process (though not always). If you don't want to take the chance you could be waiting for a while, you have the option to pay for settlement priority processing. If you have a straightforward application (ex. no previous visa refusals, no entry bounces, etc), your visa will be processed in 15 business days or less. This service is an additional $300.
As you've been married and living together outside the UK for 4 or more years (and can prove this), you have some options available to you.
You can go to the UK before you apply for your visa, take the Life in the UK test (Knowledge of Life, KOL), and when you apply for your visa, include the test pass certificate with your supporting documents and request Indefinite Leave to Enter (ILE, also known as permanent residence). The downside to this option is that you'll have to pay for the cost of the trip to the UK to take the test.
If you can't take the Life in the UK test prior to applying for your visa, you can get a spouse visa with a KOL REQ endorsement. This lets you take the Life in the UK test and apply for Indefinite Leave to Remain (ILR, same as ILE) as soon as you're in the country without waiting for a 2 year residence requirement. The downside to this option is that you'll have to pay for an ILR application.