And one burner is broken on the current cooktop. 

I've talked to the local appliance shop that it was purchased from and they said it is more than likely caused from misuse or incorrect care there examples were dropping some thing, sudden change of temp by placing a cold pan on a hot burner, overheating the glass, ect. 

Maybe they want a stove with a power burner.Different burners have different BTU's--maybe they are using the simmer burner vs the power burner--if the stove has one.Did install one of these, that work well with only one type of cookware (???) 

A 5% vacancy is  the same as 1 turnover every 20 months. 5/100=1/20. 4units*$1000=$4000/20 months = $200 per month and that doesn't cover clogged toilets, broken burner or leaky faucets.

But putting that question on the back burner for now, here's the scenario I'm considering: If I do choose to rent it out, I have some work I'd like to do on it, including a bathroom renovation and tiling in the kitchen.

All 3 water heaters are electric and toward the end of their usable life. scenario 1: Total $19,4001 new gas boiler, $6K2 gas burners to use on existing boilers, $6K3 new water heaters, $3,600chimney liner, repair, waterproofing etc, $3,800 scenario 2: Total $17,6253 new high efficiency wall mounted combi units $16,500chimney work $1,125I'd appreciate any insight regarding this process, the high efficiency units, maintenance, reliability, availability of parts, ease of use and repair or if there are any other alternatives I should be considering. 

What repairs will be done(clogged toilet, burner not working, hvac service, etc).

All 3 water heaters are electric and toward the end of their usable life. scenario 1: Total $19,4001 new gas boiler, $6K2 gas burners to use on existing boilers, $6K3 new water heaters, $3,600chimney liner, repair, waterproofing etc, $3,800 scenario 2: Total $17,6253 new high efficiency wall mounted combi units $16,500chimney work $1,125I'd appreciate any insight regarding this process, the high efficiency units, maintenance, reliability, availability of parts, ease of use and repair or if there are any other alternatives I should be considering.

(This may or may not be a good draw though--I was just conceptualizing a nice glass of wine in front of cozy fire in the wood burner on a cold winters day).Good Luck!

I would put this on the back burner.