If you buy through EFI (link found on my first post) you can get a decent ~53% efficiency 4" x 48" for about $600, (delivered), which is far cheaper than any manufacturer's buy-direct or the Home Depot online price at any similar performance point. (You can open an account with EFI over the phone with a credit card to get their listed price, or at least that was my experience a few years ago. I've bought other goods from them since- they're a decent vendor, good communication, no markup on shipping, always very professional.)
They need to be mounted vertically for soapy drainwater to have sufficient surface tension to spread out for good heat transfer. Anywhere downstream of the shower's drain flow is fine, as long as it's dead-straight vertical. They also need to feed both the cold-side of the hot water heater AND the cold side of the shower to deliver the rated performance. It's usually easiest to plumb it in next to the water heater, but it'll still work from 50' away, if need be.
A 199,000 BTU-in condensing tankless can only be counted on for about 190,000 BTU/hr out. At 6 gpm you're running about 3000 lbs/hr, so the max temperature rise you can get from incoming water to shower-heads is 190,000/3000= 63F. With a 105 F showering temp, that means the lowest incoming water temp you can tolerate is 42F. If you like killer-hot 110F showers you'd need at least 47F incoming water.
But if you add the drainwater heat recovery it just won't matter what the incoming water temp is- even 32F glacial runoff would be fine, since it would be raising the temp to about 70F ahead of the water heater, giving it HUGE margin on burner output. It also lowers the pressure drop you'd normally see across the tankless a bit too, since it's using proportionally less tankless flow, and higher cold-side flow. The difference in pressure between the hot & cold due to the heat exchanger of a tankless is sometimes an issue for anti-scald mixer valves, particularly when run at higher flow. A condensing tank heater doesn't have this issue, and even the 76,000BTU/hr Vertex tank is enough burner to run nearly-endless 5gpm showers when used in conjunction with drainwater heat recovery. The 100,000 BTU burner version would be as endless in your application as the tankless, with none of the tankless quirks, and fewer maintenance issues to boot.
If the heat exchanger needs to be tucked into a stud bay, the 3" x 60" versions can usually be made to just barely squeak in with a bit of adjustment on wallboard thickness, but it's tight. A 2" x 60" will fit in a stud bay easier, but only delivers ~45% efficiency at 2.5gpm, maybe 35% @ 6 gpm. (Which is enough, but not the slam dunk a bigger/fatter version would be.) Where the existing drain runs from the second floor down to the first is the most likely placement, unless you have a full basement to work with too. Simply feeding the whole-house cold distribution with the heat exchanger works, but it means the cold water draws from other taps will also be 70-75F whenever someone is in the shower. (Which is room, temp, eh? Most people will tolerate room temp cold water for most uses, since that's the temp the water in the cold water pipes stagnates to, no matter what the incoming water temp is.)