I second what Bodge said - go for the biggest hoop size you can afford.
I bought a Husqvarna Viking Topaz 25, and I am very happy with it. I chose Husqvarna Viking because I was used to a Pfaff machine (Pfaff is now HV). And also because Mr Acorn has a Husqvarna Viking lawn mower, and I thought it was funny.
Mainly, though, because at that point it had the largest sewing area for the price (150mm x 240mm). I was lucky enough to get a practically new used one from a dealer with a massive price reduction (I paid £800 against a list price of £1599). It had been returned by someone almost immediately after purchase because they wanted - guess what - a larger sewing area.