There are primarily two problems with the Brompton, both problems are lumps of squishy stuff. One is the lump of squishy rubber that forms the suspension block, and other is the squishy bit between pedals and handlebars.
The former, even with the most firm of the offerings from Brompton is not firm enough, unless you are either a) bloody light, b) not carrying much. Stick 100kg of dyke on a brompton, add 10kg of luggage, and even with 16 gears, it wallows about like a pig... You could argue that this use case is actually more a flaw in the 100kg of dyke, rather than the 50g of rubber... but it's something many don't realise. My Brompton has a 1.85m lowest gear, and a 8.18m upper gear. I have had cause to use all of these (65kph down hill on IoM was fun, 5kph up hill, less so...) My preferred gear for pootling around town is the 4.83 (38t front, 20t rear, 5th gear of the 8 on the SA hub), I have no idea what that converts to in archaic measures.
As for the mud guards yes or no. YES! Then you can fit an easy wheel to it, so you can wheel the bloody thing down the train platform, or round the supermarket. Better yet, stick a rack on so it's more stable to push round the shop. Rack + easy wheels is the best upgrade you can make on a Brompton for usability as a town bike.
J