In reality, selling the home, taking the equity and buying better investments might be the best route, but we have great tenants of about 6 years and I probably have too soft of a heart. 

Rotten means you would be able to push a screwdriver into the soft wood.

I find most owners tend to be soft and let lease violation slide way to often - especially when they live on site and have to see the tenant's every day. 

Pre covid-$175 sq ft for construction for 3 and/or 4 bed/2 full bath 1250 sq ft apt's in a triplex and $25/sq ft for all soft costs-architect, permits, SAC, park fees, landscaping, and various other soft costs ect but not including the cost of a standard sized lot.

Around the toilets and bath tubs, feel the floor to see if it is soft, this is an indication of rotted wood floors, big issues.Roof look for double layer of shingles or age. you can look in the city, permit search, for the last time of replacement, if they tell you is new ask for the permits.Sewages and water lines, yes Cast Iron and black pipes (mostly commercial) can be an issue but look for the clean out and how far it is from trees.

What are all the hard costs and soft costs associated with this?  

sellers should know how to do this.if its a sale to owner occ at the minimum Soft staging on higher end full staging like we do. 

The issue is they are super soft, and one storm will break it down the middle or at the trunk if it's still small- avoid this one! 

I would be inclined to order a deck cleaner/wash, a deck paint of your color preference, and general paint/staining supplies (brushes, rollers) from a nearby Home Depot/Lowe's, pay for them, and have them available for pick-up by the tenant at the contractor's desk. 

Another fairly easy step is a local engineering firm, they can give you the soft cost as well as the hard cost estimates, at least a ballpark per pad up front.