Can you weld? if so it's not that hard to knock up a winch bumper. the main thing thats needed are STRONG chassis mounts, the stock holes in the bumper need to be strengthened, either by welding some tubes into them or plating the chassis rails or you can use U-bolts around the cross member that bolt to the bumper (or even both). and always use grade 8 or higher bolts.
You'll find most winch bumpers have some sort of stinger/Abar on them to mount spot lights too anyway.
Also most winches come with the roller fairleads so you don't need to worry about that, I personally want a goodwinch tds 9.5c as you really dont need a 12000 for an sj because they dont weigh much more than a ton, well unless you plan on winching out stuck land/rangerovers all the time

a smaller winch also weighs less so thats an advantage. If you can afford it I would also upgrade to a dynemma/plasma rope to save even more weight. I like to keep everything as light as possible as that is the SJs main advantage over other trucks
But yep I would say thats the sort of budget you need, £350-500 for a winch & rope (unless you can find a good one second hand) then £50-£200 for a bumper depending on whether you make or buy one.