West Coast is better OP, Flic en Flac is amazing, gorgeous beach and there are lots of bars and restaurants in Flic, I'd recommend the Maifozzo Bar. Port Louis is very busy and not the prettiest of places, although Caudan Waterfront is gorgeous.
If I were you, I'd stay four nights in Flic, load of hotels, pic whichever takes your fancy, or air BnB it. Then I'd head to Grand Baie for a night or two, that's the tourist Mecca in the North of the Island, very expensive but very "western" if that's is more comfortable for you. After that I'd head down south to Le Mourne and get a few nights down that end of the island.
Driving is the same as the UK, but there are speed cameras everywhere and youll be amazed at the sheer number of police around, they do a lot of road checks so just watch your speed, which is in KPH for your info. Hiring a car is easy, PINGOU are based at the airport and are very reasonable, no hold on your card either, and their cars are in pretty good shape.
Don't stay on the East Coast, it's very windy, great for water sports but not for lounging by the pool.
Don't worry about dress code, just don't walk around in your swim wear and you'll be fine.
For currency, its usually cheaper to just use an MCB cash machine, honestly, the exchange rate from an MCB cash point is about as good as you'll get anywhere.
Have fun OP