The megaforce will fit about 750-1k points,IMO.
A XV8 can also be a HQ(with improved stats,check the codex),hence you got 1 HQ 2 Troops right there.You might or might not field the gun drones and pirahnas depending on preferences and playstyle,those aside pretty much everything will be useful in a normal game.
While the megaforce is a good start,you'd still need to buy a lot more stuff though.The most important being Kroot more battlesuits(of all 3 varieties) and more Fire Warriors.After which there's Devilfishes,Hammerheads and Skyrays(only buy Skyray kits,they come with the parts for all 3 vehicles so all you need to do is change the turret and you're good to go).Lol it never ends.Let it not be said that this is a cheap hobby.
And before I forget,welcome to the Greater Good.And get that codex ASAP