Hi, my nephew and I did this trip in January in a 20 year old Mitsubishi Shogun. Had no real problems other than with the car. Diesel in Tunisia was pretty cheap and in Algeria it was less than 15 pence a litre thats £10 to fill her up.
There is pot holed tarmac to Tam, and the road to Silet is ok.
It's a bit awkward to find the piste out of Silet but with a guide you'll be ok. Once you're in the desert its mainly hard flat and fast.
We never bothered with a guide after Bordj. and took the wrong piste to Tessalit in a sand storm but eventually found the road. Then we drove straight into the military base thinking it was Tessalit and ater a bollocking got escorted to the Douenne.
At the border in Tessalit you can pick up cheap fuel smuggled in from Algeria if you are low. We picked up a military seargeant of some kind wanting a lift to Gao, which was not what we wanted but came in handy as there is a big military presence up there and the piste into Gao is a bit dubious.
I wouldn't mind doing it again, apart from the expence of the guide in Algeria (65 euro's a day) and also I,m planning to drive from Banjul to Accra and possibly on to Bamako this January.
Anyway, all the best.