Where to begin...
My initial thought is that based on my experience with a whole-house gas fired tankless is that you might be better off with a small tank unit.
But since your requirements are rather different, it may work out. Plus, I'm assuming you are speaking of one of the small electric units.
First off, what we've found with our unit is that it does not maintain the set temperature well. I don't know if that's due to a fault, or the system's design or what. But when set at 122 F it varies from that up to scalding. What it means for the darkroom is that you can't rely on what the faucet will get. I have a temp control mixing valve which could possibly handle it, but that leads to factor 2;
2. Tankless heaters need a minimum flow to operate, my ground water is about the same temp as yours, but even so, getting to 68 degrees or so, doesn't require much hot water. For things like print washing and film washing, the demand isn't enough to cause the heater to come on. Our unit needs about .5 gpm to activate, as I recall.
This will probably be the major consideration for you. For print washing, I compensate by turning on the hot water for a bit so the pipes are charged with hot water, then letting the print washer run off what's built up, but after 10 or 15 minutes, the water is ice cold. It helps to do print washing while doing laundry. Since you're only using it for the darkroom, however, additional use probably isn't an option.
I just looked at the small units Home Depot sells, but they don't list the minimum flow requirement.
3. The third problem area is that our unit seems to delay restarting by a few seconds if you stop the water and restart it. This means you get a slug of cold water in the middle of the heated stream. For the darkroom, that may not be so bad, but it's a PITA if you're showering or washing dishes. Tankless units may save energy, but they do not save water. I don't know how the small units will react to intermitant use. For the darkroom situation, I don't think it would be a major problem, since the use isn't very intermitant. But it would be something to watch out for, especially for film washing, if you aren't using a good temp controller.
4. Tankless units are sensitive to scale. If you don't feed it softened water you'll need to de-scale it with vinegar periodically. Also, it will dump pieces of scale into the water (since there is no tank for it to accumulate in). You WILL need a hot water filter between the heater and your faucet.
If you go with the tankless unit, you will need a pair of these valves to accomodate the de-scaling;
http://www.plumbingsupply.com/isolationvalves.html
My humble advice is to not let the plumber talk you out of them.
That said, I've been considering a small electric tankless unit to serve just my darkroom. My space is very limited and it would be a lot easier to accomodate.
Your mileage will vary, but this is a good description of my experience so far with the tankless world.
If you ever consider a whole house unit, Fine Homebuilding magazine did an excellent article a couple of years ago on integrating a whole-house tankless heater. One of their recommendations is to combine the tankless unit with a small tank unit to accomodate the low-flow and intermitant use situations. The article should be available on their web site, though it mostly won't apply to your specific plans.
Finally, wer'e spoiled here in the U.S. Tankless water heaters have been common in the rest of the world for some time.
P.S. If the unit you are considering varies the water flow to regulate temperature (our whole house unit does), a pressure sensing mixing valve like what is common for showers will not work well. It will end up fighting with the water heater, and the manufacturer of ours (Bosch) recommends against them. A temperature sensing unit, like the Arkays and similar is ok, but more expensive.
Yes, retro-fiting a gas fired unit is extremely expensive. For ours, the installation cost as much as the unit, and we under-spent. Heating water from 50 degrees to 120+ while it's flowing through the space of a large camera case needs serious heat, and serious gas flow to do it. The BTU rating on ours is close to double that of the furnace that heats the house.
Some of the small electric units run on 110 though. Since the darkroom water heating demand is modest, that may not be an issue.