the rules changed a few months ago so that even if you stayed at a hotel which should file tm30 for you there is no need to refike at your local immigration when you get home. I have read this and ut has been confirmed by my local immigration. That doesn't mean that all offices will do the same. If you Google tm30 you will probably find this information.
you should have a certificate of insurance both for the compulsory and additional insurance you have bought. That should explain what is covered. New cars are the most expensive to insure. I bought mine off Mr Prakan a broker who was very helpful. My vehicle is 20 years old and level 2 insurance cost 7000 baht. Your cost seems low for the most comprehensive cover.
I needed a letter from the bank confirming funds which was 100 baht at CW. I also needed letter from my embassy requesting that the transfer was made. UK.
I rented a car for 18 months and the rental company checked to see if their insurance would cover me on my UK license and/or IDP. They said they would accept both. My IDP ran for two years. I subsequently obtained a Thai license just to be sure.