I converted my 2018 Specialized Crosstrail Sport Disc with a hubdrive kit from Dillenger Bikes. It was an easy one day project. The kit cost $750 and had every part I needed - except the Dillenger kit required a Freewheel and the Specialized came with a Cassette/Freehub so I couldn't move it from the donor bike to the electric. Also the Specialized has a disc brake rotor that is a center mount style and the Dillenger wheel uses a disc brake rotor that affixes with screws. Both were minor items that cost less than $40 for both combined.
It works great and I'm happy with the bike. I've put over 600 miles on it since the conversion in January.
A Mid-Drive might be even easier to convert as you use the Specialized wheel in stock form. Mid-Drive kits cost a bit more and are just as easy to convert but you need a few special bike tools to remove the crankset and bottom bracket of your donor bike.
As to battery mounting. You can put it below the downtube and use hose clamps to hold the battery mount to the tube. Like this: