Go and try the bikes you can afford – at least ride them round the block.
As pitduck says – the spesh is hard to beat.
You will need a helmet and gloves plus a reasonable waterproof.
As you know it gets wet up there. :-)
Spare tubes and pump will make alot of sense too(when you puncture),
so have a look for a rucksack/camelbak to carry them in.
(You could just fit them to your frame and use a water bottle 'til
you have the extra readies).
Have a look on'tinternet for prices or ask the bike shop if they'll give you 10%off accessories when you buy the bike from them.
btw, some great routes around Skiddaw too…..
use the search facility on this forum..
Most importantly have fun.
J.