no motor, Are you referring to spacers on the axle or spacers on the freehub. Big difference. If you have spacers on the freehub (I'm not referring to the spacers between the gears of the cassette), then you have a 7 speed cassette that was mounted on a 8/9 speed freehub and you are really in luck. All you would have to do is replace the 7 speed cassette and additional spacers with the 8 speed cassette and your other parts and you're good to go. BUT If you have the stock rear wheel (silver Alex rim/Shimano hub) for a 2000 Hardrock, it came with a 7 speed cassette/freehub and the 'spacers' the mechanic was referring to are the spacers on the axle that center the wheel. To upgrade that wheel to 8 speed you'll have to replace the 7speed freehub with an 8/9/10 Shimano freehub (use a 10mm hex for that) and you will have to try different spacers along with re-dishing the wheel to center it (been there/done that for the same bike).
If you do have to make changes to your wheel, now would be a good time to think about replacing that wheel if you were inclined to do so.