HRMs are possibly the best piece of kit you can buy if you use it properly. If you're on a tight budget you just need one that reads your heart rate, so you can check your zone.
If you want to spend a bit more, get a better brand, but don't bother with ones that calculate zones for you, unless you go top end, as the cheaper ones just use formulae that are worse than ones in much cheaper tri/cycling books. If you've got a Decathlon nearby, theirs are cheap and appear reliable, or try Ebay or the internet for a new one from there.
Start with a cheap one and see how you go, but once you've used one properly, you'll probably wonder how you ever managed before. Personally, my HRM is the highest spec piece of kit I have, much better quality than my bike, wetsuit etc. After all, your training is the most important thing you do. Spending and extra Â£500 on a bike may give you 3 mins over 40k, but learning how to train more effectively and productively will give you much more for much less money.