So, Im torn between buying a Peavey 6550+ head or an LTDEX400/EC500 and some kind of Fender standard Stratocaster (dunno if Id get a SSS or HSS model). Any thoughts?

So, you're actually choosing between buying an amp, or two guitars?
I've never heard of a Peavey 6550 actually. I only know the 6550 tubes.
I can only talk about the guitars...
I'm not a big fan of Explorer model guitars; most of them sound like crap in my opinion. Only the original Gibson Explorer sounds pretty good. They're annoying to play while sitting, but ... ofcourse, they look cool
If you're buying LTD, I would first of all recommend going for an ESP, instead of LTD, but if you're still going for LTD, I'd really buy the EC500 model. They are pretty heavy, but because of the heavyness have a really nice sustain and 'ballsy' tone.
Fender Stratocasters still are one of the best guitars on the planet.. Unfortunately also one of the most popular. Because of this, there are THOUSANDS of shitty Strats out there. The only way to buying a good Strat (or Telecaster for that matter) is visiting lots and lots of stores and playing on them, and listening to their sound. One day you'll find one, which sounds great and plays great. This is why most guitarists stumble across their best guitars / Fenders by accident.
If you're going for a Fender, think about buying a custom shop and / or relic. They are unbelievably expensive, but sound and play as good as guitars come.

Good luck with the guitar / amp hunt, and ofcourse, when you buy something, I'm very curious about the pictures

Woops, the amp is actually a "6505," not a "6550." My mistake. Its the same thing as a 5150, if youve heard of/played one of those.

As much as I would like to get an ESP or Fender American series guitar, I really dont have enough money for them. But yeah, Id definitely be trying out everything before buying it. Of course, when I do actually decide and buy 'em, there will be pictures and probably some clips.

I know what you mean. Playing guitar, and making music in general is a really expensive hobby, or profession for some.
Even though it's expensive, I always tell musicians and some of my students to buy the best gear they can with the money they have at the moment, or have patience and save more money and buy the thing you really want.

I guess I'm lucky that I get my ideal sound through hacked together, semi-broken, oft-improvised gear then.