Modern road hubs are 130mm, not 135. 135 is a mtb spec. While you could choose to use a 135mm hub, why? If you go with a road hub at 130mm, you don't need to respace the rear end unless you really want to. You will easily be able to pull the dropouts wider (it's only 4mm) each time you shove the wheel in.
Also, how badly out of true are the old wheels? Based on the bad advice you've already had from your mechanic, I'd get a second opinion at another shop and see if the wheels can be retrued. If not, the cheapest option may be a prebuilt wheel but you might want to rebuild using the existing hubs, assuming the bearings are still good.