I have the 17 inch S-Lites with run flats on my mini at the moment but am looking to get something more eye catching. Im thinking of going with either 17 or 18s but not too sure which. What are your thoughts on wheel size (width and depth), is it good going to 18s or does that sap a bit off acceleration?

 

Search for the many many past threads on various wheel sizes. Pros and cons have already been discussed at length.

Any wheel wider or larger than stock 17x7 rims are going to-
Work with smaller sidewall (often stiffer) tires
weigh the same or more (although stock rims are very heavy)
Have less clearance and chance for rubbing if tires are wide or too tall, or if offset is not the right amount.

17x7 is safe but you can consider 17x7.5 or 17x8
205/45-17 is safe, but you can consider 215/45-17 if you run stock suspension. For lowered MINI consider 215/40-17 for more clearance.

18x7 is OK with offset about +40 to 45mm, 18x7.5 or 18x8 can work but with a little less offset about +38 to 40mm. Tires need to about 24" tall tire diameter or risk rubbing. 205/40-18, 215/35-18, 215/40-18, and some wider sizes but you need at least a 7.5" wide rim.

Anytime you get a wider rim you usually add weight, you get more rolling resistance so usually less mpg and a wider contact patch so usually better grip and handling when cornering.

Added weight from tire or wheel isn't a good thing. Your Stock wheels are already very heavy so any upgrade would be an improvement.

Many owners like the look of 18" wheels, realize that you may need 35 or 40 series tires to go along with that rim, whereas 17" wheels may work well with 45 series tires.

If your roads are very bad then lower profile tires aren't recommended or be very careful driving or you can damage your expensive tires, rims or suspension.

Try going to your local MINI club events and look for tire and wheel combos that you like then go for a test drive and see what you think.
Realize that the ride quality is a combination of suspension and tire/wheel selection, on the roads of your area. Smooth roads can make almost any wheel combo feel pretty good.

Have fun shopping.