As long as you get the games onto your download list before they expire you can access them for as long as you maintain your subscription - not just while they are the currently offered games.
Anything you buy at a discounted price is yours permanently regardless of whether you let your subscription lapse.
The PSN store has periodic offers on subscriptions (discounts and extra months free). There is a 30% discount on the 90-day membership beginning on the 6th of February but if you wait until then you'll miss out on Bioshock 2, Ratchet & Clank All 4 One and CubixxHD.
See the following blog post:
PlayStation Plus: Your February content – PlayStation.Blog.Europe