1) Mauritius is a small island although u'll be staying in the East at belle Mare plage hotel I assume, it takes 1hr to reach the other part (towns and city) so yeah there are a lot of places of interest and it depends on ure duration of stay
2) There are a few car rental services available and they are represented at the airport and in the hotel as well
3)It takes 3-4 hours to tour mauritius provided ure sticking to the coastal roads
4)Hmm dunno much about the petrol though but I guess for a <1500CC car, with Rs 1400 (22£) , u've filled ure petrol tank!
5) Try ure hotel guide but its gonna cost ( the best thing tourists usually do is they hire a private taxi and have the guy showing them the top spots but make sure you don't get cheated...have an approximate cost in mind already discussed with the hotel staff)
I hope this helps Browse in the forum as I am sure some of your questions have already been answered in the archives