I was thinking something like:
Conjure Fiend
level 5 conjuration v,s, m (see below)
casting 1 round, duration 11 hour.
When using this spell, you indicate to the DM which type of fiend you are trying to conjure: devil, demon, other (includes yugoloths, succubus/incubus, hell hound, nightmare, night hag, rakasha)
Roll a 1d8. This is the fiend you get. If the CR of the fiend is equal to or less than level you cast the spell at, the fiend is friendly toward you and your allies. If the CR of the fiend is higher than level you cast the spell, the fiend's attitude is indifferent and the fiend will request an offering (see below). If the offering is made, the fiend's attitude will be friendly. If not, it will be hostile (and remain for the full hour). If the caster loses concentration, the fiend will become hostile regardless of whether an offering was made, and will remain for 1 hour. If you leave the area after casting the spell, the fiend will attempt to get a sacrifice from anyone else in the area. The fiend will be to detect where its caster went, but won't be inclined to follow unless the sacrifice is made to it by someone else. If no one sacrifices to the fiend, it will be hostile toward everything it encounters for the rest of the hour.
1-roll again. If the caster chose devil, the conjured creature will be a demon (and hostile). If the caster chose demon, the conjured creature will be a devil (and hostile). If the caster chose other, the DM picks between devil and demon (either way hostile)
2-CR 4 fiend (closest lower level if CR 4 does not exist)
3-CR 5 fiend (closest lower level if CR 5 does not exist)
4-CR 6 fiend (closest lower level if CR 6 does not exist)
5-CR 7 fiend (closest lower level if CR 7 does not exist)
6-CR 8 fiend (closest lower level if CR 8 does not exist)
7-CR 9 fiend (closest lower level if CR 9 does not exist)
8-higher than a CR 9 fiend (horned devil, nalfashee, or arcanaloth)
I will leave it to others to come up with the appropriate sacrifices. The temptation of this spell is you might get something really powerful. The problem is that if you can't pay for it, you will have to fight it. Of course, if you cast it and dimension door out of there, it will be someone else's problem for an hour.......
Casting at higher level doesn't change the fiends, but it does increase the chance that the fiend will be friendly towards your group without sacrifice.