Have you rung them and paid extra for fully inclusive, just did a dummy booking and it wasn't an online option (although I only looked quickly) online bookings usually only give basic cover which isn't enough.
This is what it gave me when I clicked on car hire:
Car hire
Car hire is included in your total price. Please note that car hire only includes basic third party insurance cover. We strongly recommend that you take out full insurance to cover any potential damage to your hire car. Please contact us before you travel to upgrade your insurance.
If you have paid the extra then you are fully covered.
Sat nav, I bought one about 3 years ago in
Walmart for £35, their website says from around £50 now but they are sometimes onoffer instore.
http://www.walmart.com/search/search-ng.do?Find=Find&_refineresult=true&ic=16_0&search_constraint=0&search_query=tomtom&search_sort=4
Only searched Tomtom, other brands might be cheaper.
Other option is to watch ebay, people buy them in Walmart, use them 2 weeks then sell in UK, sometimes for a profit. They sell from around £30 or with quality/latest maps from around £50
http://www.ebay.co.uk/sch/i.html?_sacat=0&_from=R40&LH_Complete=1&_nkw=tomtom+us+maps&_sop=15