I went to Koh Lipe the same time last year and must say it is a very pretty island however with very little life at that time of the year. We stayed at mountain view resort which is at the other side of the island compared to where all the restaurants and shops are. It has an extremely beautiful view and a dessert paradise island - however it is quite expensive and the staff is not the most friendly you can find. Anyway, I didn't care as I was not there to chat with them ;) So if you want picture perfect beaches and some peace and quite Koh Lipe could be a good choice however it takes a while to get there. I think Railay Beach is just as nice though and with your budget you could stay at Ton Sai beach and just walk over or take a longtail boat to the pretty beaches at Railay. The food on Ton Sai is so much cheaper than on Railay and the atmosphere is more laidback however the hotels are mostly budget.

We love the Andaman Sea area and have done several 3 week trips there, one being for Songkran.

Our round trip normally starts and ends in Phuket for cheap flights and includes

Phuket-Phi Phi-Lanat-Krabi-Khao Lak-Phuket. I certainly wouldn't want to add in any more destinations as travel between places can be slow. You should try and spend a little longer each stay to really get the feel of the place.

Take a look at Bob and Janette's Travel Adventures for some ideas as they seem to travel that area regularly. Lots of pics of beachside places