Hi Stu welcome
I am also based in the UK and have converted a mill currently working on a lathe.
The first correct thing you have done is come here for advice, this will save a lot of heartache.
I also have a lot of experience of woodwork.
Firstly you say "can you cut 18mm mdf in one pass" can you do this manually ?
If no then this will also be no with cnc.
CNC is for repetition with minimal user intervention, it is easy to spend hours programming cnc only to know that you could have done it quicker manually.
Motion Controlled products are good and i use them myself but i do no recommend kits. The MCP BOB is very basic, checkout www.cnc4ps.com for BOB's plus other stuff etc.
Speed is a combination of things, firstly stepper motors lose most of their power above 1000 rpm "Try To Keep Below 1000 RPM" so with a 1.5mm 1.5 x 1000 = 1.5 mts per min pitch thread you will be looking at a top speed of 60ipm, Not Very Fast at all.
Second and this is why i do not like kits relates to speed again the motors in that kit will be fine but i would use higher voltage power supplies and higher voltage drivers again from MCP
Think of this simple formulae Stepper Motor Speed = voltage, Stepper Motor power/torque = Amps
With lower voltage, Stepper Motors will not rev high they stall even with the motor shaft not connected to the machine, that equates to lost steps which is scrap parts and wasted time.
Now if you change to ball screws with 5mm pitch the above scenario would give you 5 x 1000 = 5 mts per min or 200ipm IPM is mainly used on here.
At 5mm which i use, i can hold a tolerance of .005mm so you could use 10mm pitch ballscrews get 400ipm with a tolerance of .01mm
Now you may think you do not need 400ipm but you need to move the table even if not cutting so that's where you need that extra speed on big tables.
The other advantage of ballscrews is friction, it is minimal the other is rigidity and lastly accuracy. The downside is cost.
A ballscrew will also come in various grades this is given in say .001" per foot so the max error over the length of the screw is known this is not available with standard studding.
So does your kit come with that option "Ballscrews" if no then the kit will have a lot of wasted parts and you will have to find a manufacturer to make the parts but this will not be cheap.
Another solution is plans, that way you can mix and match what you want i think a lot of builders here use Rockcliff, ask another question.
Hope this helps get you on the way.