The Express has a substantially weaker signal than the Extreme. Hence the limited range. Also, the Extreme is dual-band and now offers 3x3 MIMO. The result is that the Extreme offers higher data rates to more users at farther distance and with more obstacles /interference in between.
I have an Express from 2005 too as well as two Extremes. No way I'd use an Express in lieu of my main Extreme. Entirely different league.
Oh and regarding 2), absolutely. No doubt. However, there is no guarantee a new Extreme will solve all your issues if your Express suffers from interference close to its new location. That's what you should check first. Microwave ovens, cordless phones, large conductive materials (metal panels, radiators, appliances, etc.) close to the Express, etc. are things you don't want close to your wifi base station.