Hi Sophie,
Your tank has a capacity of about 85 litres, which fiits the recommended capacity for the fluval U2 (45-110 litres) so it would do the job for a tank of that size, the only thing to note is that all internal filters of this type tend to produce particularly enthusiastic water flow. A spraybar will be essential to keep your axies happy.
I would personally recommend an external cannister filter as they're generally gentler and I've found them much better in regards to cleaning and maintenance. They are, however, substantially more expensive than internal ones.
The general rule of thumb is 18 inches square of floor space per axie, so I'd tend to agree with Kerry on the space issue - a wee bit cramped but not too bad. I keep four axies in a 4' tank and they're perfectly happy in there.