For the money, it will be a fine introduction. Heavy, but fine. Components will be weighty (note the steel bars). Thorn also do a good range, but may be more expensive. I have a 700C and a 26″ Dawes, and there isn’t a lot in it to be honest. I’ve also not had much problem with standard V brakes or cantis, but I don’t ride long dragging hills, so rim over-heating hasn’t been a problem. Upgrade the brakes when you’ve ridden them, but bear in mind that the shifters are integrated, so you might like to go down the route of BB7 cabled instead of buying new shifters.
Yes, you’ll need the large. And probably a new stem. Adjustable stoker stems come highly recommended too.
You could try Warlands Cycles in Botley, Oxford. Dealers in Landscape tandems (Pete Bird’s latest venture). See if they can supply something at budget. They have a lot of experience. As do Locks of Sandwich.
They are great fun, I just need another stoke or two (kids grew up, wife can’t cycle any more).