I've been a liveaboard for 30 years and believe me its quite a lifestyle change if you are going to liveaboard full time.
To buy a boat you need to look long term, you may be as fit as a butchers dog now but time and tide wait for no man. Choose a boat with easy access, not only for getting on and off quickly when mooring, but also mundane things like putting bikes on and off, loading your shopping on board etc. Whatever boat you buy there's one golden rule, happy wife = happy life, if not 2 things will happen, either a divorce, or the boat will be sold, the only difference is the time it will take. We have a 42 foot ex hireboat single engine motor cruiser, originally an 8 berth now it's comfortablefor the two of us, with a relatively low freeboard, the reason is simple, advancing years and access, we're retired so we monitor the weather and only go to sea when conditions are ideal. Having said that, we have, like everyone, been caught out with force 4 touching 5 seas and our boat proved it's ability to cope, (p.s ginger is good for preventing sea sickness) with any engine you need to put preventive maintainence at the very top of your list and strive for nothing less than perfection.
There are many on this forum who go way over the top with unecessarily complicated systems, my advice is to keep it simple. Someone mentioned fridges and freezers, the simple fact is that the liveaboard lifestyle is not so energetic as shore based so you won't need massive household fridges, you either reduce your food intake or you'll become overweight, go for quality, not quantity. When you fit a fridges or freezers ONLY fit a frost free one, save yourself (or your wife) the hassle of defrosting. Fit a Danforth/Secop 220/110-12v auto changeover so that when you are in a marina shorepower will take care of that and when cruising your engine alternators will carry the load, or in case of shorepower breakdown. We use a 4Kg Aquamatic washing machine which is sufficient for our purposes, for drying clothes do it the old fashioned way, use solar and wind power, hang it on a line. You'll need a maximum efficiency battery charging system, we use a system made by ADVERC in the UK which has never let us down, when on shorepower we have a Victron double outlet system, the main outlet is for charging domestic batteries and a secondary outlet trickle charges the bow thruster batteries. We recommend fitting a hydraulic bow thruster a size larger than recommended because electric ones have a thermal cutout and if you get caught out by the weather the last thing you need is the darn thing to quit. (It happened to us and we had to revert to old fashioned boat handling skills). Early electric models with brush motors covered everything thing around them with carbon dust so avoid those.
If you lead a hectic shore based life and picture yourself anchored in lovely bays reading a book and sipping a glass of wine, yes its nice, but beware because after a while you will get bored with it and that can lead to tetchy relations. We don't anchor out as the admiral gets a gippy tummy from the motion, we don't touch alcohol when we're moving the boat.
You also need to be aware of your power consumption battery size, recharging. When buying appliances check out their consumption ie a fast boil kettle will consume 2.2kw whereas a low consumption kettle would be around 950w, many marina's have a power limit on shorepower and in the winter its easy to use too much power and jump the trip switch
Remember the 5 'P'military maxim. Proper Planning Prevents P*ss Poor Performance.
If time is not pressing, visit any hire boat and secondhand boat sales you can find, take a clipboard and camera, note photograph all the good points and especially any bad points to avoid.
Finally everybody thinks they can easily drive a boat, they can't there's an art to it, go to a reputable boat handling school and learn how to do close quarters handling properly, the same with an engine maintainence course.
Its a wonderful life if you adapt to it. when you've stopped working your income will drop so plan your future life accordingly.