I am a regular visitor to Thailand but I was left feeling disappointed after my 12 day holiday there last September. Whilst we did enjoy three or four fantastic days of sunshine and blistering heat, the remaining days were ruined by heavy rain and constant showers, which meant that you really couldn't plan anything in advance for fear of it raining! July to September are the wettest months in Thailand and best avoided, especially if you're going there to spend a fortnight lying on a beach.
I guess it all depends what you want out of a holiday though. Being such a special occasion, why not up your budget ever so slightly and buy a round-the-world ticket, stopping off at a few cities of your choosing - including Bangkok! I am sure you can pick these up for �800 or �900 and if you can make do with 2-3* accommodation, the trip would certainly come in under budget.