Dictas and other shitty unbranded freewheels are available on eBay from around £3. They will generally perform and last as you'd expect given you've spent less than the cost of a decent beer on a pretty major component of your drivetrain.
Shimano freewheels seem to be very hit or miss these days, one person says they have one that's fantastic, the next says theirs is shite. Given the price I'd not consider them worth the gamble.
Halo etc, I've never tried one because for a few qiud more than they cost new you can get a used...
White Ind. It's pretty much impossible to explain how much of a difference using a WI freewheel makes, I never beleived the hype then I bought a used one and it was a revalation. Yes they cost 10x what a Dicta costs but it'll last you 10x as long and it's also way nicer to use.
As for gearing, add a tooth or 2 over your fixed cog, you don't need to worry about spinning out on descents as you can just coast so you may as well give your knees an easier time on the climbs.