I have a level 13 priest and I was wondering if it's worth switching
to a lord for the extra AC and training him in some close combat.
Also, if I do will I still be able to get level 7 spells
at level 18?

As of now I've maxed my divinity and the magic realms I'm only going
to use. Therefore, I don't have anything worthwhile to put my SP's
into.
My stats are:
str: 65
int: 90
pie: 88
vit: 70
dex: 45
spd: 65
sen: 40

I don't think you'd get level 7 spells at 18th level; believe it would be 5 levels of lord before you'd start learning new spells as for their first 5 levels they don't cast spells. So I think by level 23 you'd get them?

Probably I would stay a Priest with the charcater until 18th if you want 7th level spells, then switch to Fighter so you can do beserk attacks. Try to work on his Str before then. Also not sure why you went so high in INT, for divine spells PIE is the most important stat.
The stats he has are more suited to a Bishop.

Make it level 22. The lord has 4 levels without spells.

But then you can cast level 7 spells only relyable at power level 1. Thats not very useful.
Level 14 seems ok to me to switch to a fighter. Then you can use the level 2-4 buff spells at high power level. If you wait longer, i would keep the spellcasting ability, and stay priest, or become lord (mainly for the infinity helmet IMO)

For a pure (mainly spellcasting) priest, int is very nice, since the realm skills (fire ...) raise much faster.
But for a priest who wants to melee a lot, and even switch to a lord or fighter, int isn't important.