Assuming combined basics of 65k , keeping car loan 183pm , no other loans and under 35 years of age, all the Lenders would do the 245k for you both at 100% and 92%
Lenders that do 100% are PTSB , FA, UB , BOS , ICS (EBS and AIF for professionals).
There is no best Lender, check out their current rates and product offerings before deciding who to go with.
With regard the site you have, if you own it generally most lenders will allow you to get 100% of the cost of construction assuming your salaries can cover that amount. (BOS don't do site builds though). You'll need full planning permission on the site to begin any build project and a certified builder\architect to supervise construction.
Are you building second home or extension? If second home, do you intend on selling first one or renting out?