I think you get what you pay for in general. The more expensive kits come pretty well finished meaning you don't have to do that much work to tidy up from the manufacturing process, you just have to bolt it together into a car.
Cheaper kits that I have seen don't come as well finished or as accurately made, so you might find yourself tidying up edges and even trimming areas where the panels don't fit.
You also need to think about what you want at the end and what type of build you want to embark upon.
Some kits are based on a single donor car, the idea being to break a car and use as many parts from that car as possible. The RH is one of them. This seems like a good idea at first until you consider the engine you'll end up with. You can do a lot better than a Pinto or CVH these days, but this will be the choice you have if you base it on a sierra. Other parts from the single donor won't be ideal, but the kit will have been compromised to use them. Of course you can use parts from other cars, but as you deviate from the single donor costs and time will be added.
Consider also if you'll want to refurbish every part from a donor and use it. To do so saves money but will take much more time. If you see yourself buying refurbished parts and otherwise deviating from the single donor path, you may as well throw away all the compromises that this involves and build something that draws on the best parts from several donors.
Consider also the use it will get. Do you want something to polish and display on concours? Something to use mainly on the road as something "a bit different" to drive on sunny days? Or, do you want a track day weapon?
If I were doing it again?
For a track day car I'd probably go for a bike engined seven style car or a powerful tuned 4 pot (Ford Duratec / VX C20XE / 1.6 Ecotec). I'd maybe venture towards an Audi 1.8T.
For a road car I'd seriously consider Westfield's single donor kit based on an MX-5 (Donor car with a more lively engine, lightweight parts throughout and fun to drive in the first place). Either that or I'd use a Ford Sigma engine (little Zetec from the Fiesta / Puma, seriously light and loves to rev). Or I might consider trying to shoehorn an X32SE into one for a bit more of a cruiser ;-)
For a polisher I'd go for something with a bit more bodywork to it than a Lotus 7 style car. Maybe a Cobra or Healey 3000 replica. It'd rude not to have a V8 in a Cobra.
For a money no object all rounder have a google for the Ultima... I was speaking to someone at Le Mans who's not allowed to go to Belgium any more thanks to one of these. He did try (and succeed) to run for the border after getting clocked a little over the limit (205 MPH).
The main thing is to see the cars in the flesh and decide what you think. Look at a RH against a Caterham and you'll see where the compromises have been made. It's not a slim, svelte little car, it's got a bit more lardy to accommodate the Sierra running gear underneath unmodified, whereas the Caterham uses mostly bespoke suspension parts. There are other manufacturers who are at various points between these two extremes.
Most manufacturers will give to a factory tour, talk about their kits and give you a test drive in a demonstrator. Westfield will happily throw you the keys to a demonstrator and let you play for half an hour or so. Others prefer to take you out and scare you.
It's a shame you're not around at the weekend because at a kit car show you will find so many shapes and sizes of car it'll really open your mind.
Cheers,
Kevin