School bit first, as this impacts the house bit.
I can imagine which two schools you have applied for. The assessments for both schools are not rigourous academic ones if he is due to enter yr 1 or yr 2 in September - both schools will take him round, have him spend time in class and make sure he is (a) someone likely to fit in (and if he's a run of the mill child that won't be a problem) and (b) someone likely to enjoy it. If they do do a test it will only be to make sure there isn;t a significant issue.
What might be a future problem is that there is a big step up to yr 3 in one of the schools (the more expensive one) and if he were to go straight in at that age to the school then he would have to sit a full-on entrance test. But if he is already in the pre-prep they don't do a test any more - there are only around 11 children in each class so they know the kids inside out.
Sometimes, if a child is bright, but is behind (eg due to dyslexia) they may suggest the child stays down a year but in your case, even if he was behind in terms of schooling I am sure they'd just get him up to speed quickly - I've seen them do miracles with children who don't speak any English at all. It's pretty unlikely they'd suggest a repeat year.
So.... unless you think your DS is really a bit thick then the schools will all be fine. If he really is not even vaguely bright (only you can tell), then you may want to avoid the more expensive school because there is a (very small) risk that they may suggest he move to another school at the Yr 3 stage (to my knowledge this has only been done once in last 3 years, and that was for a child with SN who was clearly struggling). AFAIK neither of the other two private schools would move out/refuse a child on purely academic grounds.
The reason that impacts school is because the more expensive one has a minibus system (I don't think the other ones do - could be wrong) which pick up children (age 3 upwards) from the outlying villages. It's a godsend if you want to live further out than the city centre. It only runs in the mornings but is very handy. Boughton and Faversham for example have both been mentioned and I am sure they are on the bus routes. The school can give you details.
Where to live - Faversham, Canterbury West, Wye and Ashford are the key stations to go for - other train stops link into these stations but you have to switch trains/wait/take slower trains which can significantly add to the journeys. So I'd advise you choose within commuting distance of one of these stations. Boughton is a nice village as someone has mentioned already. Faversham is OK too but I prefer the villages rather than the town centre. Whitstable and Herne Bay are also other options. Wye is a fabulous choice (there is an escorted train service for kids to the more expensive school so you can drop DS at the station and the school has it's own carriage on the train with teachers accompanying the kids)
If you choose a state school then Blean and Selling become good village options for a house - and I am sure there are others. The good schools might however be oversubscribed (esp. Blean).
Although I bought a house round here when I lived abroad without any problem, I'd recommend renting first. It could be a long hunt if you are looking for a rental, though, and you will need to snap something up the minute it becomes available - price isn't a problem but availability is - I reckon about 1/3 of the parents at one of the schools are non-locals who move to the area because of the school and the train links to London, and all of them tend to want to rent initially. Demand is high but your budget gets you a very good standard of housing if you can find the property. Whitstable is probably a good option because there are lots of holiday homes there and downturn may have meant people needing to rent them out instead of using them themselves.
The schools you are looking at both have newsletters, so once you pick the school you could ask if they'd put in an ad for you asking if anyone knows of a good rental property - the school parents are the families who will be moving out of nice rented family properties because they have settled into the area and bought their own.
Blimey. that was an epic.