Hi,
I'm not sure that there are any legal limitations (up to the limitations imposed by the license itself. The Meridian and the PC12 are both turboprops which in JAA require a type rating. You can only get a type rating after getting a license.
Practically though (speaking as an instructor) larger aircraft are generally harder to fly for a number of reasons:
1. Inertia. Think how difficult it is for the big oil tankers to manoeuvre. It takes ages for them to stop / change direction etc. This means that the pilot has to anticipate the need to change direction well in advance. Conversely, a canoe can turn on a sixpence.
2. Systems. Large aircraft (almost) always have more systems. Even the simple ones have more stuff going on to bamboozle the pilot. In stead of simply learning how to fly, he also has to learn what to do if the landing gear doesn't retract, what the limitations are if a suction pump fails etc.
3. Speed. 95kts (110mph) may seem pretty slow... until you try navigating for the first time! That's basically 2 miles per minute. As a PPL, you're allowed to fly with 3km (about to 2 miles) visibility. Since you're only navigating by what you can see out of the window, it can get ugly very quickly. Larger aircraft generally go faster. Oh, and they have many more "limiting speeds" for example the max speed you can operate the flaps on a C152 is 85kts for any flap setting. On larger aircraft it may have different limiting speeds for each flap setting. So there's much more to remember (going back to systems)
4. Ruggedness. Traditional training aircraft are generally designed to take a hammering (and trust me, you'll give it a hammering with more than a few of your landings to start with!) They're also going slower, so any impact is slower (I'm not talking about crashing into stuff, but dodgy landings which aren't quite straight)
5. Airfields. Big aircraft need big runways. Part of the fun of learning to fly is going to obscure places.
In short - walk before you can run!
So which aircraft is best? I've instructed on C152 / C172, PA28 and AA5.
The Cessnas are great reliable little workhorses, but I don't like the high wing as it gets in the way of a good lookout.
AA5 is great to fly, it just feels like you're connected to the aircraft. They've also got a great view. However they're less reliable than the rest so the cost is higher. Also, there's not many of them around, so after learning on that, you'd have to do a few hours difference training on something else.
My favourite's the Piper. Plenty of room, good view, comfy (except when it's raining... but that goes for most old aircraft!) reliable. Oh, and they're available everywhere. Their success is for a reason!
I can't comment on any of the new fangled wizardry that's available as I haven't flown them (Diamond, sport cruiser etc.).