Welcome to the wonderful world of Swallow Boating Jim!
I was in the same position as you last year when I put a deposit down on a Baycruiser 23 at Excel and went through the buying process through the year for final delivery at the Southampton Boat Show. This was my second new Swallow Boat I have bought (the other being a WE BR20), so I have a degree of experience with the Swallow Boats yard - I'll gladly offer any advice and help I can, and if I don't know I can guarantee someone on this forum will.
My first and probably the best advice I can give is to email laura@swallowboats.com (Matt's PA) and get her on board, copy any emails to SB to her - she gets things done and will keep you informed re the progress of your boat.
Be as certain (as you can be) on what you want fitted on the boat - and ensure all is listed on the 'spread sheet' which the lads doing the actual building refer to. Go through that list of 'extra's' with a fine tooth comb, try to establish what is actually 'standard' on the boat - I found there is a bit of blurring between the two! 'Extra's' may also be things you consider essential! The costs go up exponentially with 'extra's' especially if you want something that is neither 'standard' or an 'extra'.
The fit out of your boat is a very personal thing and dependent on what you want to do with her. But if you do want fancy electronics I would urge you to go for one single manufacturer for all bits - makes life a lot easier for the builders (hence cheaper for you) and generally will be plug and play so they all work together. Again there is a wealth of advice on here re electrics. Don't forget nav lights/steaming/anchor if you are going to be out at night - although expensive for the yard to fit for you is far better done at the outset.
The trailer is a major factor (as is the engine), ensure it is a braked trailer (do not be fobbed off with the idea a standard unbraked (750 kg) will be ok. Hence it is not cheap - assuming, of course, you need a trailer. I haven't met anyone yet who doesn't need one). I found the cost for Matt to supply spare wheel and bracket to be better than I could do myself so don't forget to order those with the trailer.
Soft furnishings/cushions - I personally found the ones suppled by SB were not particularly brilliant and an 'upgrade' was silly money. I took my boat after delivery to a local upholsterer and got substantially better and cheaper.
The subject of engines is vast and there has been many discussions here. It depends on where you are sailing. On a lake/reservoir a small 2 - 3.5 hp is fine. The ones without a reverse gear are I think, worse than useless.
My opinion is that you should go for a good 4 or 5 hp four stroke, (manufacturers models and price are fairly standard) with proper forward and reverse gears. It must be short (standard) shaft, ideally with a sail drive prop if you can run to that - if you are hoping to make progress in any kind of sea or current then you will be glad of it. Don't waste one second of your time thinking of electric outboards (torqueedo's) etc, no matter what others might say. Again, they are just not feasible - unless again you are just lake sailing.
I could go on all day! But I hope this is a start for you - if you have any queries just ask!
Kind regards
Andy
Baycruiser23 No.25 'Equinox'