Well if you truly want to get the most out of your money (squueze the most out of every penny), only buy with AP, AP-exclusive items, like capsules, Iyan 09 armour, et cetera.
edit: as lurusk added, if you're truly a money-grubber (just kidding) only go for the best capsule gun weapons, such as the FG42 (insanely buffed M14, barely any recoil whatsoever, HS machine), G36 Alligator, AK47 Predator, or if you're a sniper, go for the Mosin Nagant, that's pretty much a FR-F2 with a scope that has no zoom on it. /edit
There are some euro-shop guns that may be worth buying with AP, such as the top-tier ones, that are unlocked at MSG1, i.e. the Para rank. Such as the Para (obviously) and the FR-F2. The other top tier guns, like the M4 MK3 and PGM, that are unlocked at SFC1 (light blue), are sorta worth buying with AP if you can't be bothered grinding exp, or waiting until you rank up, but it's really not that hard to reach SFC1.
That "spend 1AP" event, if you want to be cheap, buy a 1 day Heartbreaker I grenade, it's only 20AP, and it's the cheapest item in AVA. If you were planning on buying a gun with AP in the first place, before the event was announced, you might as well go for the PGM Freedom, because it's limited edition, and you were probably planning on AP'ing the regular PGM anyways, and you could buy the FR-F2 with euros some other time
If you do go for the PGM, you won't be disappointed, put on the +15 DMG barrel on, and the precision scope, you don't need anything else, grips and triggers are purely optional.