There is a huge variation on the fleet, so impossible to say exactly what model they'll be using in almost a years time. Yes, M group is guaranteed sat nav, but generally charged at a huge premium to an equivalent car without navigation. I always download maps to my smart phone before travel - costs nothing - just as effective.
£300 for 16 days rental over Christmas? - that's a 'too good to be true' rate. Did it include insurance? If it is real, guaranteed, and from Avis (rather than a broker who may or may not have accurate rates loaded) I'd grab it.