I think XP, how quickly it's given out in a "standard" game, how quickly players level, how the different xp requirements for each class affect the game, how the game plays at different levels with characters who started at 0 versus characters who started at 5, etc. IS a concern for playtesting.
In honor of the "old-school" theme I was going to assign 1xp / hp; with a bonus of +1 xp/attack die and +2xp for each extra power. Just to establish a base line; I can dial it up if the players think its too slow.
Is exp going to be "shared" or do you only get exp for the stuff we kill? I ask now because I know my players will ask.
I used the Basic D&D rules, which are similar in value. I share all exp equally to group survivors (level 0 henchmen and hirelings get half shares, level 0 characters for a player in a funnel get full shares if they survived).
I think you will find that this will be too slow if you don't also give 1 exp per gp of treasure found, and on average provide treasure worth 2 exp for every 1 exp in combat value of the target. The session I ran Thursday the group happened to not find much in terms of treasure so I've seen what happens if it is just creatures. Despite defeating 20 creatures with 1d6 hp each in two battles, and killing and driving away two 2 HD crocodiles, and contesting with a trap and guardian that they ended up running from, they still won't get all that much experience. I'm okay with that for one session, but you don't want to have too many sessions where the players are actively engaged for three plus hours and only come away 35 exp each (tentative award, still figuring a few things). I'm definitely going to house rule that if you had a funnel party of level 0 characters you automatically get to level 1 at 0 exp after a survived adventure of that length.