You are correct: you will not pay stamp duty because the threshold is £250,000.
I assume that by "Northeast" you mean NE England -- not Scotland. I have bought and sold four properties in England over the past 35 years and it can be very stressful indeed. A contract is only finalised at the 11th hour and, until then, the buyer or vendor can pull out, change price or delay. Sadly, I have found that people are not always telling the truth. For example, vendors may claim to be "chain free" but, after a few weeks, you learn that they really haven't committed to a new property. Or the person they were buying from also has decided to find a different place. The chain can grow and grow.
To get a good idea of prices, look at everything in your area/price range on Rightmove and Zoopla and see how quickly they've gone under offer, or if they've been reduced. Check the sold prices. I've noticed that lately the Land Registry (
https://www.gov.uk/search-house-prices) has more recent sales data than Rightmove. Visit local estate agents and explain that you're a cash buyer who can proceed quickly. Often an agent will know about a property that's stuck in a chain or is about to come on the market. When you've found a property, be sure that you get a structural survey and a good solicitor. Be aware of issues that a mortgage company might question and raise these yourself.
In your situation, I see SELLING as a bigger problem because it can take months to complete. If buyers know that you're about to return to the States, they could ask for a reduction at any point.. As long as you own the property, you must pay Council Tax, utilities and homeowners insurance. And, remember that FX rates may go against you when you want to change the proceeds back to USD.
Good luck.