We bought resale a number of years ago and were happy with the process, but it is a little more stressful and time consuming than direct. Although we bought our resale during the BCV pre-close sales - so it actually wasn't more time consuming - but that was a rare sort of situation.
I'd say count on more like twelve weeks before you can make reservations - eight if it all goes smoothly, but there seems to always be some "oops, this didn't get notarized" or "Disney hasn't done ROFR yet" or "we have a backup in closing" with a resale - twelve is more realistic - and that assumes that the sale goes through (Disney exercising ROFR twenty seven days after you make your offer is going to set the whole thing back) and that your seller is in the U.S. (People who buy where the current owner is overseas have longer timelines due to mail delays).
300 points PLUS developer points is going to be a lot of points - have a good plan for them. Also, I haven't been paying attention, but I have the impression that not everyone has been satisfied with the usability of the developer points - might be worth a search or even a post about it to make sure they are going to be usable in the fashion you think they will be.
The only different four months from now (after the stress of "when will I be in the system!") between your resale contract and a new contract is that with a new contract you'll have a bag or something from Disney. Disney treats resale owners the same - buying and getting into the system may take longer and be more stressful, but once you are in, it will be the same).