I've never played an Imperial, but I've got an Indiana and a Medalist and love both of them. They're almost identical apart from the engraving and the crook on the Medalist being longer and thinner - they both sound wonderful and blow easier than my Conn and tolerate all kinds of different mouthpieces and a lot of sloppy playing.
Some people may find the older style keywork not to their taste, but I like it - I've got small fingers and the stretch to bottom Bb on a Yamaha is awkward for me.
It's the tone of Martins that attracts their fans- even Stephen Howard, who's not a man to hold back on criticisms of instruments failings, has some favourable things to say about Martin saxes -
http://www.shwoodwind.co.uk/Reviews/Saxes/Alto/Martin_Handcraft_alto.htm
while this review is of a far older model than the one you're talking about, his comments about the tone and intonation hold true for most Martins - my Medalist is a bit brighter and more responsive than my friend's 1920's 'The Martin' (similar to the one in the reiview), but it's still a warm sounding instrument, even with my ultra edgy Lawton 8*BB mouthpiece and me screaming at full blast it's still got a fairly fat tone
£210 is a decent price and if you don't like it you could sell it for that much or more
Vintage saxes aren't for everyone, they have their quirks and imperfections - Martins don't have such good intonation as modern instruments, but if you work at it a bit they will play in tune.
If you want perfection then a Yanagisawa or Yamaha will be a better choice, but you won't get one of those for £210.
Check for leaks and damage before you hand over your cash - a full overhaul and repad won't make it such a bargain..