Personally, i’m a big fan of the dedicated Singlespeed hub, but it’s by no means necessary. They are stronger in theory but, as Breadcrumb pointed out, it’s hardly a game changer.
Are you looking at retaining your existing alfine setup? If not, you could look at buying somethinig like the DMR hub and having it built onto your existing rear rim. Even factoring in the price of the new freewheel and having the bike shop build the wheel for you, i think you’d be looking no more than £120.
Failing that, you could buy a ‘normal’ multispeed wheel and some spacers.