Strahd's tough because he's both an awesome warrior (in his past) and a powerful necromancer (in his present) and a Ravenloft lord w/ all the powers that come from that. (Not to mention he's had many different power levels throughout D&D).
So partly this depends on which aspect you want to accentuate, as well as the hit points you'd like. There's a good chance a Fighter base won't give you enough necromancy and a Wizard base won't give you enough h.p. or defense in general (especially since you're supposed to reduce h.p. w/ the Vampire template due to all the resistances & fast healing). So you might have to build Strahd like a monster, from scratch.
In general, a monster can mirror many of the same maximums a Fighter PC can have as well as some "appropriate for level" special abilities that a PC wouldn't gain access to. But they wouldn't have the full selection of spells or consumables or whatnot.
It takes some mechanical mastery which is rare without the GMG out yet, but you can improvise (and maybe it'll be out by the time you need him).
1. Choose a level, likely PCs+2 to 4, depending on how many henchman he'll have and how much prep/forewarning/attrition the PCs will have.
2. Build a Fighter at that level-1, with basic magic equipment, but without feats (except maybe one you think suits Strahd) and no critical specialization (again, unless one suits him) and none of the extras from higher proficiencies except the numbers themselves (so no Evasion for example, even if he's a Master at Reflex). Start him with "the best" stats, so 18 in Str/Int (being his shtick), 16 elsewhere.
3. Take the stats, i.e. AC = 29, and keep them even though you take away the equipment. And assume he has Toughness and raised all his stats at 5th/10th/etc. For example, if faux Strahd w/ his magic weapon has a +25 attack for 3d12+Str damage, he'll keep that bonus even w/o a weapon and keep the same average damage (19.5 + Str). Don't forget extra damage from runes too! (which might come through as negative energy or just added as basic physical damage)
So more likely +1d6 to that for 23 + Str average.
Except he's using claws (right?) which are d8s.
So likely 4d8 (18 average) + 5 + Str for Strahd's damage. And yes, the +5 comes from nowhere essentially. It's just to mirror a comparable Fighter. You can even add +3 MORE damage to represent as if he'd taken a Barbarian MCD! (Bestiary numbers mirror these options I'm mentioning, so you're still within balance.)
4. Retcon Strahd's basic stats. So if you'd given your Fighter-Strahd a +2 weapon, maybe give him a +4 Str (which is +2 in monster terms since they only use modifiers anyway). So a 20 Str Fighter w/ +2 weapon would be like a 24 Str monsters, except in PF2 a monster would use "+7".
This is true for Saves too. So maybe bump his Dex, Con, & Wis to account for the bonuses from his armor (which is gone now), but don't change the final value of course.
5. Add the Vampire template. This is why I said choose target level -1 because this adds another level.
6. So you have a fairly stripped Vampire of appropriate power w/ several of the "True Vampire" abilities thrown in from that list. If that's enough, then yay!
7. But he's a lord!
Well that's troublesome, isn't it? The simplest thing to do is give him virtual immortality (via respawning) and control of the Mists. That shouldn't change his Level. If you want his super-Necromancy too, that'll be tough. I'd take a few select spells suitable for his level (as if he was a Wizard w/ few castings) and let his Skills & Rituals represent most of his power. If you want a full-caster, then lower his hit points/level (from Fighter build) to 8 instead of 10 (before max Con & Toughness) and maybe give him Harm for self-healing. Then add even more spells to his list, like Horrid Wilting if he's 15th+ and so forth.
8. Look at comparable monsters in the Bestiary to see how well your numbers are aligning. Adjust as you think fits best. Heck, if you want him to be 12th, you could re-skin the Lich. :)
9. Rest. That's a lot of work for one critter. :)