Would have to know more about it.The best leasing info in my area is from Costar.
You can find out what everyone else is paying,class of building and amenities,and when their leases are coming up for renewal.
When you buy this building are their any height restrictions for redevelopment??
Often the class of building and requirements expected by the city or the county can greatly ramp up building costs and delays.
How large would the new building you want to build be approximately??
When the developers I have worked with do a tear down a place with asbestos and other things costs much more to demolish.You would think you would just tear it down but they want all that remediated and removed first so it doesn't go airborne during the tear down.
Building cost 5 years from now is impossible to speculate.Labor and materials could be way up and LEED demands might be a requirement then.
Can an additional level be built on the existing structure if it is beefed up at the bottom?? Then you can just reface the outside and update the interior.
The city and county over time also change how they view development.In five years you have different people in office which might be pro-development or anti-development.
So there is a ton of factors that play into it.Right now you can land great contractor teams in commercial for cheap.Once the big jobs come back around it will be harder to find quality work for a small job such as you are proposing.
Hope it helps.