Frank's got a turn-key 48v package which is very neat - give him a call. Would be quick, easy to fit and come with a compact handlebar-mounted LCD display to regulate power modes, cruise control button etc. for a grand to £1,200 depending on battery if you want it all on a plate to fit in an afternoon. At 48v, 10Ah battery gives you 480Wh and 15Ah 720Wh. The bike will be a real performer and involve very little work.
Just to expand a bit on d8veh's post above, the 500W CST bits cost me about £550 after duty and shipping (DHL) last month with a 20Ah battery and a few extras. Kept my full range of gears (27sp) by using the CST as there's no freewheel needed.
Battery will be in a frame triangle box (making this to measure - will hopefully end up something along the lines of this, albeit in silver colourway and on a hardtail MTB rather than a full-sus -
http://www.pedelecs.co.uk/forum/electric-bicycles/12481-rocky-mountain-element-conversion-3.html#post150117 ). Battery slots between seat tube and down tube but there's space in there for controller and some extra bits like in-built lighting, accessory 12v charging socket circuitry etc. if you wanted anything like that.
Built the motor into wheel (it isn't supplied built) and installing / upgrading a number of features over the motor and BMS accessories, estimated total cost about £850 - £900 for upgraded spec / extras including in-built CREE T6 lighting (which you don't get with Xipi kit), upgraded connectors with chassis -mounted battery charger socket on the box, waterproof accessory circuit (also not on Kube kit), cable glands, in-line hydraulic brake cutoff switch (for combi-shifters - likely simpler/cheaper if separate brake levers), double-throttle cruise control, master switch (can be key or toggle), modified torque arm etc... all being specced, sourced and bought for me to assemble and install on top of the basic kit).
Would be about £150 more with Speedict or Cycle Analyst to hit a grand or so total (but I looked at both and decided not to bother for now - a cycle computer is £6 - £45 and a numerical voltmeter battery monitor about £5, with all speed management done from the double-throttle cruise control).
Quite a bit of modding involved and basic tools for soldering, drilling, cutting etc. may well all be needed depending on what you do. d8veh has been providing a load of help to put it all together, some of which is up on here but more besides owing to my inexperience. Could cut out lots of bells & whistles to simplify and shave down costs considerably (e.g. get triangle bag in place of a box, use cheaper materials etc.) ... but you can have it all the way you want it within reason with the full self build approach ... with the notable exception of a handy LCD if no Speedict or CA used - and with the exception of a top-end programmable controller.
Top-end programmable controller to work with that setup would set you back £200-£400 (depending on whether Cycle Analyst is included) in addition, and likely some longish delivery delays.
