I think its fine to expect people to pay their own way, those that cant afford it wont come. As for your parents, of course you would want them there so if they need help you probably would have to contribute a bit towards their fares/accomodation. I wouldnt be expecting gifts though, in fact you could even look into providing a welcoming gift in guests hotel rooms as a thankyou for coming sort of thing.

I also agree that booking your flights & accomodation via the net is much cheaper than going through an agent. However as for the wedding, many of the hotels have wedding planners so that could be handy for you.

How exciting, hope it all comes together for you & you have a Thailand wedding.