Dear,
1- Get vis for Umra
2- Get letter from ur freind allowing you to travel by his car (from traffic dep.)
3- check for ur car oil, tyers, filters...etc
4- Befor leaving Doha, make lining for the car to avoid scratches by sand on highway (u may use liquid soap)
5- As this is ur first time, the best time to leave doha befor 12:00 Noon time, if u start from Doha at 12:00, u will be on boards at 1:00 pm, u will spend around one hour finishing papers in qatari side and saudi side, then u will start the trip inside saudi at 2:00 pm to reach Hafouf around 3:30 pm,take rest there, then to Riyadh. take care, the road from boards to Hafouf -in some parts under construction and the road will be 2-way, so you need to be carefull and that is wahy I suggested u to take this part during day time.
6- there are a lot of rest point on the road, avoide driving when u r tiered, take a rest then go on.
7- try to be relax all time to enjoy ur trip, it is great chance to power yourself, so no tension.

if u want to ask for anything more, please let me know