AD600B has full manual options (1/1 - 1/256 in 25 steps), TTL and HSS
AD200 has full manual (1/1 - 1/128), TTL and HSS.
There are also the AD200Pro, AD300 Pro, AD400 Pro and AD600 Pro
Finally there is the old AD600BM which is an AD600B with only manual options, no TTL or HSS.
(You probably want to go with the AD600B or AD600BM, rather than AD600 or AD600M, if you chose an AD600 as these are the Bowens mount versions - Bowens mount being the most common mount type for reflectors, softboxes, etc, and so the most cost effective).
If aiming to overpower the sun, then go with the AD600B if you can afford it - even in the UK you need plenty of power (There is even an AD1200, but that's £1474.99 on pre-order at Lencarta, so well over your initial budget!)
The other option, which would give flexibility, is to look at the AD-B2 twin adapter, which allows you to fit two AD200's side by side in a single Bowens mount bracket, giving 400Ws (you can also use it with just one AD200 as a Bowens Mount adapter) - while this is less power than a single AD600, it does mean you if you are in a situation were you don't need the high power you have two lights.
All of which just adds more options, I'm afraid
Note: If you are intending to shoot outdoors, you need to be aware that large softboxes and umbrellas will be prone to going where the wind wants to take them!
I would advise looking at the Lencarta Profold softboxes - they are both quick to assemble / dissemble due to the folding mechanism (you definitely want folding softboxes if you're going to move them from site to site), but have D rings at each corner (a feature I've not seen on any other softboxes), which means you can attach guy ropes to them to guard against the wind if needed! Not the cheapest, but good quality.