I've got a Meguiars G220. I can't fault it tbh but it's not cheap. Look to pay around £200 for one (£250 for the polishes and pads as well)
There are other Random Orbital Polisher on the market, but most are non UK and require a transformer as well. I would go for a Random Orbital Polisher as they are a bit more forgiving in the hands of someone starting out in the world of machine polishing. The last thing you want to do is knacker your paintwork by polishing through the lacquer.