I'll keep this short and add more later as I'm stuck for time
I was there for about the same length of time last August, flew with Turkish Airlines to Bangkok (think the tkt cost around €650ish, anyway it was cheaper to call in to the office in Dublin and collect by hand than purchase over the web). Bangkok is a lively spot but it all depends on what you're looking for, I only stayed for 3 days and then flew to Chiang Mai for 5 days, a bit more relaxed than Bangkok. Did the ziplining, night-safari, night markets etc, a search on here and google should throw up suggestions for you. After CM I flew back to Bangkok and got a bus from there to Koh Chang (around 250 bahts not including the ferry, took about 6 hours). Really liked this spot, but not sure how old you are so you might prefer one of the islands in the south as they're more lively, if that's your thing. From here I went to Cambodia but I'll stop here as it's not relevant to you.
As for shots I got Hep A, Hep B, Tetanus, Polio, Diphtheria and Typhoid. Some of them were multiples in one, 3 in 1 / 2 in 1 etc. I got these done in the Travel Health Clinic on Dawson st, good service and very friendly staff. Can't recall the price however. If you're getting the Hep B shot you should enquire soon as you have to get 3 over a few months.
As for malaria tablets, they were recommended but I didn't bother in the end. You should def get a strong mozzie repellant and cream to treat bites as they itch like hell!
Accomodation wise, check out agoda.com, some really good deals on this site and there's an iphone app which was really useful to me over there. I booked in advance mostly, but towards the end of my trip I booked a day or two before I arrived at various destinations without any hassle. Check out Travelfish.org, it's full of really good info on Thailand/South East Asia and was a great resource during my trip.
Travelling alone, it depends what you end up doing ie. falling around drunk on your own at 4am might not be the most sensible thing to do. But if you keep your wits about you I can't see why travelling alone is an issue...! It wouldn't be for me anyway
Whatever you do make sure you get travel insurance, it's not worth the risk of going without...
Finally, in terms of budget, again it depends on what you're doing. You could get by on €1000 and have a great time (that's not including flight btw), but you could really live it up on €2000 ie. stay in really nice places, drinks and dinners out each evening (out as in a restaurants and not from a street vendor, not knocking street food either - anything I had was really really good!!), internal flights and ahem... indulge a few ladies to a wink and a drink!
You can get toiletries easily, don't bother bringing loads with you.
Don't bother bringing too much clothes either as you can get stuff very cheap over there.
Bring sun lotion.
Don't make the mistake of getting a bag that's too big as remember you'll have to drag it everywhere with you.
Bring an mp3 player with some good tunes
leave the ipad and laptop at home, not worth the hassle!!
Don't use internet banking over there or any other sensitive and personal web usage. I brought a thumbdrive with me with portable firefox (portableapps.com)on it and some software like no script etc, worked well for me. I don't trust any browser outside the EU
If you don't want to carry cash around you can load your credit card with it and take it out over there from an atm, check with your bank on charges, I'm with BOI and I think it was either a reduced charge or no charges when I had credited money on my card before withdrawal over there.
More useful sites:
If I think of anything else I'll add it later