I'd probably recommend a fairly basic one unless you are looking to do some very complicated training.
Provided you can set heart rate limits (upper and lower) this is about all you need. This will allow you to run at your aerobic rate, do 'threshold' runs at your anaerobic capacity and also monitor your recoveries. This is pretty much all most people will need one for.
You should be able to achieve all this for not too much cash.
Polar do some really good entry level models for quite cheep prices, not only are Polar one of the best makes on the market but they are also compatable with most gym CV equipment, ideal if you do cross training.