March or May will be hot but sunny pretty much all the time, October/November will have a lot more rain generally.
£500 spending money for a week
You could do it no problem but you'd be watching the satang every day and I'd look to rule soapies out of your itinerary as if you're going to be on a tight budget you can get more bang for your buck elsewhere. Mind you you say you don't drink and that's always my greatest expense (I easily blow £500 a week on booze alone in Pattaya). A soapie a day in Pattaya will cost you upwards of £265 over a week even if you don't tip and just nurse a bottle of water whilst making your selection. Not a lot left over for evening entertainment out of your £500 after that, booze or no booze.