This is good advise. Only thing I can add is that prime summer beach weeks and ski weeks are still very expensive to buy into (i.e. 10k at least per week) and owning at those resorts or having access to the DC (which you will not have) are the only ways to get those prime exchanges.
If a summer beach week or ski week isn't what you want, then you can buy any Marriott week and I think you will be very happy. I would sugget you buy a platinum lock off and those can be had for very little (less than 2k and that includes closing) and will give you the best trade power.
If you want a summer beach week or ski week but don't want to pay those top dollar buy in costs, probably best to buy a gold week at any of those resorts and hope to trade into the higher seasons. This can work better than buying a platinum week someplace else and trying to trade in. You get trade preference when trying to trade back into your home resort. However, gold beach weeks are still much more expensive than the mormal Marriott resale week by about 5k. Not sure how much more a gold mountain resort would be, but is proably next to free.
As far as how things work in II for trading, if you own a platinum week and trade for like size, I think you will have the best luck. If you lock off your platinum week and try to trade the studio side for a two bedroom this can work great for an instant exchange but will end in disapointement many times via an ongoing search, if you are trying to trade into prime season.
Good luck. Lots and lots of opportunities.
I am in the same boat, I have just sold a few weeks and bought a platinum Shadow Ridge two bedrooom, and a platinum Grand Chateau two bedroom, for II trading. I have a few simple rules. First, I assume any initial investment (price paid) is a loss I can afford to take - if I have to someday, I will give away the weeks for free (or even give an incentive to someone to take them away). Second, I remember that the only guarantee I have is that if all else fails - systems change the rules, exchange opportunities diminish, etc - I must be happy still using my home resort.