we are required to buy ezeds from TG, and we pay them online for our tkts. they charge currency conversion fee plus the small tkt fee from our ezed site. Read the thread about same, posted earlier.
If you have paper tkt, it's different since you buy those from AI.
Take a look at maninseat61 for a train trip from BKK all the way to KUL and SIN--there is a link for itinerary, costs, etc. It's a very impressive site--you can also go the other way to Siem Reap--just look for the link for it.
As for hotel bookings--take a look at think2travel--often you can get last minute deals off-season.
so many reasonable options in Thailand. SIN will be $$, so be prepared.