jim, ... I'd buy from Starwood where I want to go to use with a retro. Same, but even more so, for the week to be retroed. In the end, all you get when you buy a timeshare is a deed to the unit and a guaranteed use week (fixed) or a use week in a season (float).
All other benefits, SVN trading, II trading, procedures for both, II preferences in branded systems, rules for internal and external trading, Starpoint trading and use, StarOption value and Elite status, are not guaranteed and subject to change or elimination.
The worst things you can do are: 1. buy more timeshare weeks than you can easily afford and 2. buy timeshares in places or seasons where you wouldn't want to wind up vacationing.
However:
A Plat Lagunamar EOY 2-br costs about $21.5K with MF of about $640 per year. It yields 148,100 StarOptions every other year. IMO, that's the lowest cost MF for a Starwood new purchase and use for a retro.
There is very little information on prices Starwood is selling re-acquired weeks for. My experience is that the Plat EOY 2-br weeks are selling for less than $20K. So, they cannot be purchased from Starwood with a retro included. I believe Starwood has adopted that pricing to promote sales of new units. The facts and pricing of Starwood's selling re-acquired units is kept under the radar.
If you want to be able to drive to where you will vacation and want to stay in the US, the lowest cost mainland places to buy a Plat EOY 2-br for slightly over $20K are either WDW or WMH. Same StarOption yield. About the same developer price. However, the annual MF are higher - around $770 per year for each (plus separately billed property taxes).
It seems to me that you are looking to get to 5 Star Elite - eventually. I would question whether getting to 5 Star Elite eventually (or in little bites) is wise. There is no reason to pay more than you have to getting to 5 Star Elite. But, if you can't afford to go all the way now (or within a year), getting part of the way to 5 Star Elite by making EOY developer purchases and retros makes little sense.
I think it is better to make the non-developer purchases now and wait to make any developer purchases (and retros) when you have the assets to go all the way to 5 Star Elite. Salty