You should talk to the local Tessaban on what is required. We did extensions to an existing house and the plans were drawn by one of the draftsman at the Tessaban. He also got the plans approved for a total cost of 10000 baht. They can probably recommend a builder but it will probably be one that gives them a kick back.
Your two biggest problems will be the electrician and plumber. Make sure you use a licensed electrician as I have seen houses up here, supposedly by a reputable building company, that had to be rewired to make the houses safe. Good luck with the plumber as they are usually just handymen.
There are also several building companies in Phitsanulok that you see advertised on billboards along the highway. You can also talk to the building supply shops regarding a builder.
So don't be afraid to ask questions of anyone that is involved.
As far as the land transfer it can be easily done at the Lands Office. Even if you already have a contract to purchase they have their own forms that have to be signed together with the transfer. Go to the LO with the seller with the cash or bank cheque. You will also be required to sign a document the the funds are entirely your wife’s and did not come from you.
But before you do anything take a copy of the title to the LO to check the validity of the title. If it is not chanote be careful as you may not be legally entitled to buy the land or build on it.