It is very hard to finance a timeshare, unless you buy it from the resort directly. If you do, you'll pay almost twice as much as buying on the resale market.
Renaissance is the only resort downtown. If you like that area, that is your only choice. You need to figure out where you want to be? Eagle Beach (low rise section - quiet and peaceful) Palm Beach (high rise section - walk to everything) or Divi Phoenix, at the end of Palm Beach but quiet.
As for fixed or floating weeks, they both have their pluses. Again, you need to know if you'll be able to commit to the same week every year.
I have many timeshare units in Aruba. If you have any other questions you can email me directly at firstname.lastname@example.org.