First singlehanded trapeze skiff

or an older A-Class or a Shadow Cat.  The A in particular will be a lot of boat for the money, just don't expect it to be competitive against the new foilers and probably best not get too bogged down in the class politics right now either.  

The 700 is an awesome boat to sail, people knock it for not being as good as the MPS. I don't think that is entirely the case having sailed both, but they are definitely very different.  The MPS is quite nimble and responsive, certainly well refined. The 700 on the other hand is a real muscle boat, its got loads of power, it's certainly not agricultural but it is less nimble than the MPS. Both are great fun, but they are different characters. Think refined italian supercar vs a corvette. Both will do a quarter mile very fast, the italian might edge it in the corners but the yank in the corvette will certainly be having a blast and might even be faster on the straights.

Just be aware you may struggle to sell a 600 again and may drop a bit of money on it to move on.

Biggest issue on 600s is the back of the daggerboard case, foil and the kicker U-bolt.

Daggerboard case and foils are easy fixes, kicker u-bolt is a bigger job.

If there is any play in the kicker U-bolt it suggests that the ply backer has got soft. If there isnt a hatch in the back of the bulkhead then it hasnt been repaired before. If there is a hatch then it has but at least future repairs wont be as costly.
Make sure that the mast has been looked after. If the previous owner has let the varnish peel and has left it like that for several years then you run the risk of UV induced fatigue and a folding black stick, so make sure its been cared for. I suspect that all boats now have the superspar mast. If its an Angel mast be careful that the reinforcements havent blown the composite.
Dont be fooled by people trying to sell a poor boat that has loads of trick take up systems. The original out of the box rope systems all worked perfectly fine and consume less rope.
Sails are pretty long lasting but bolt ropes tear away infront of the batten pockets for a past-time (the joy of a fully battened rig). Its not a big fix and most sail lofts can repair it with a decent kevlar reinforced cloth for a small amount of cash.
Halyard cleats tend to wear. Many boats runs two cleats just to make sure. Some have opted for 2:1 halyards to reduce the load in the cleat, but the original kevlar 1:1 approach is fine as long as you regularly move the knot and cleat point in the rope.
Pro-grip has a tendancy to unpeel with time and a lot of people decided to lavish the cockpit with it. As a result some cockpits look messy where the pro-grip has gone but left evostik residue everywhere. There are also some who chose to cover their boats with sex wax which again makes it look messy. To be honest you only need pro-grip on the wingbars.

Make sure you get a boat with the right wing width. In the good old days it was easy to find swaps but thats less likely now so make sure you get a boat with the right setup. I think its upto 74kg wide wings, 74-80kg either, 80kg+ narrow, but I may be wrong. Best to check the user guide or talk to Anthony York (Chunky) at Northampton as he is the class expert.

MPS will go down wind quite happily without kite. Stenhouse rarely puts his up getting to a windy start line. See report from the Tiger. I think the B14 one. This was also the advice he gave me on my third MPS sail when it was blowing old boots at the Rutland open.
There are plenty of idiosyncracies you would need to learn, but you can leave kite for a bit. Though you will want to as it is as quick as is laser slow without.