I don't know man. If it was me, I'd probably try some sort of expanding spray-on waterproof foam sealant or RTV in the areas being described by your mechanic before I'd start having seams re-welded. You start doing that, and you might fix your water leaks, but you'd then produce a lot of new areas in the process susceptible to future rust.

You might try the expanding foam sealant or RTV route up in the cowl area first and see how it goes. I'm talking about the area under the plastic cover for the windshield wipers. Take the wipers off, take the plastic cowl off, clean everything up and apply sealant to suspect areas as needed.

That is, unless your mechanic knows for sure those seams are where your leaks are occurring, but even then, some sealant might still be all that's needed.