Even a single filter will blow half your budget practically. Also, I thought iso7 had a laminarity requirement. You could probably do it for about twice that, the fan for the ACH rate is probably the largest cost.
What we did for our clean rooms was as follows:
Sheetrock walls, cove base at the bottom and sealed with silicone. Then in front of it, we used sheet metal spaced off the wall with steel stud track, hanging down from a bar mounted around the top edge of the wall. Sheetmetal comes about 12" shy of the floor and the top of the cavity is capped by more stud track. Takeoff boots on the stud track feed the blower intake through a prefilter rack. Tee off the intake goes to another rack/damper for makeup air. Keep the rooms narrow and you won’t need a floor assembly