Thanks to all the folks who've contributed here...lots of food for thought!

The Tanglewood on ebay sounds good - I generally find Tanglewood guitars quite acceptable, so if their mandolins are comparable, it'd be a good learner instrument. I'll try and find one in a shop though so I can try it out, as I'd really need to see it first. The idea of borrowing one for a few weeks is good, I just need to find a willing lender. I'll probably be using the mandolin for two different styles of music - Irish trad and oldtime American Folk (1920s style).