Thats a valid point Wongy, but pimpjuice you have to ask you this at the end of the day:
a)Do you want a car thats more likely to get you pulled or vandalised by jealous SCUM?
b)Do you want a car thats quicker, yes but you are more likely to loose your license because yes they have quick high speeds.
From the sounds of it, this is one of your first cars, i tried looking for valvers and Im sorry but I couldnt find any with low mileage, to be honest the majority have been thrashed.
To say the least your grandad doesnt sound like an 18 yr old boy racer, I reckon he has looked after that car really well and you wont have many problems.
At the end of the day in my opinion (for now) its good to have a car that will take you up to 70/80 as fast as possible.
In terms of tunning your looking around 10-15bhp increase for HP stage 1, am getting in 2weeks for around the £400 mark and then Stage 2 is £1200 you get polished port/skimmed head etc taking you to about 140bhp but the car has a lot of torque low ranger and it a quick and nippy motor and will be even quicker when you get a stage 2.
I I were you, get the car enjoy it, see what the speed is like and then decide 4 months down the line.