If he accepts peace its time to get busy building up your garrisons and replacing any lost soldiers, you'll need them against another enemy soon. Chances are he will agree, you came out of nowhere and rapidly did some major damage to his kingdom while he already has two enemies, he should be afraid of losing more.

Once you have those castles its time to see about offering Sarleon peace, send a persuasion companion out through your minister and hope it works. Unclaimed castles also make your kingdom much more attractive to any lord you are trying to recruit, its been a while, but i believe up to five are calculated into the equation. Claiming them gives no garrison, where saying you wish to defer the decision gives some free troops to the garrison! Not totally free, you have to pay their weekly wages, but you didn't have to train them. Do that as quickly as you can but remember not to take these castles for yourself. Now use the mercenaries to either aid you or simply sit in Laria as defense while you grab the castles nearby. You want to wait until Sarleon is already at war with two other factions, then quickly take Laria and hire a mercenary company.

Say you chose the area around Laria, which is an awesome spot. Already have all of that done? Great! Time for more waiting. Build up your relations with villages where you are going to start your kingdom, the ability to recruit more and higher tier soldiers in the area will come in handy. Take the time to get enterprises going everywhere as well as getting and then sending out each companion to get some right to rule.

Prophesy of pendor mercenary company