The bike mechanic doesn't know what he's talking about. It's pretty easy to fix up the bike for any kind of terrain you want to ride. The DA crank won't go below 42 teeth (assuming it's a 144bcd crank). I'd get a compact crank if you want better gearing which comes basically in two flavors.

One would be a 110 bcd compact which is pretty much the standard compact today. Sugino makes one that would look fine on an old bike and you can buy the crank from Soma fab in a 48-34 configuration.

Sugino XD Swiss Cross(48-34t) Double Crankset


The other possibility is to get a crank with a bit smaller chain ring to help with climbing. This IRD "wide compact) crank would look good on your bike and give you 46-30 chainrings:

IRD Defiant Wide Compact Road Double Crank Set (46-30T)

IRD also sells freewheels.

No big deal to fix up an old bike to get the kind of gearing and riding that you want to do. I'd find a different mechanic and/or bike shop.