I'd say do it, you're effectively getting five games at that price (MGS3 comes with the original Metal Gear and Metal Gear 2). If you're at all interested in Metal Gear Solid then you really will get a lot out of playing the old games, the backstories of each are tightly inter-related so it'll massively aid your understanding, or at least ease the confusion, if you play the old ones.
As far as the actual gameplay goes, you have to remember they are old and pre-date certain conventions; moving while aiming is tricky, you can't crouch-walk either. In fact even just first person aiming is quite tricky, but you get used to it if you do the VR training missions and what have you. The difficulty is extremely scalable from 'beatable with your eyes closed' to 'please make it stop I don't want to do this any more', there's a ton of replay value, too. Your first playthrough of MGS2 or 3 will be in the region of 20-30 hours in Normal mode, after that as you learn what to do, unlock good stuff like better camouflage or the EZ gun in MGS3 and skip all the (numerous, meandering) cutscenes you can get it down to 6 hours quite easily.
Peace Walker is completely different to any MGS so far in that you do it in small stages, there's also online co-op. My first playthrough of that where I nearly unlocked everything took over 100 hours, but it's quite grindy and samey after a while, I only kept going because I was incapacitated for over a month by tonsillitis and then a tonsillectomy and just zoned out playing it all day.
All in all, if you want to complete everything there is to do in the HD collection (except Peace Walker's codenames, because they're practically impossible) plus the platinum trophies, you'll probably be playing for a long, long time.
On the other hand, some people can't stand all the talking (there is a lot of talking, but almost all of it can be skipped) and are quite disappointed by the game-to-cutscene ratio, and I'm sure people playing it for the first time now will struggle with the controls. The graphics aren't amazing either, they haven't added HD textures or anything like that.
I should say, in case it wasn't obvious, I am a huge Metal Gear fanboy, so you might want to seek out alternate opinions before you buy, but three substantial games (if you want to replay them) for £10 is a good deal. They're not much like GTA, though, most of the time sustained firefights will result in you dying. The ideal completion method is to avoid any combat at all.