[jeff5may]

Ideally, you want to find a TXV that is rated the same btu capacity as your compressor, at the same temperature range, with the same refrigerant type, with the right sensing bulb gas, with the same size connections as your unit dictates. To put a TXV in a stock unit, the manufacturers say it must be the same as what came out or it will void your warranty.

For us, some of these rules can be bent. For instance, you can go up or down 1 size in capacity and the unit will still do its job well if you don't go out of its range. In your situation, a water-cooled condenser will develop a gob of subcooling during normal operation. This drives down your head pressure, so the TXV would open wider than it would with an air-cooled condenser to let through the same amount of liquid refrigerant. So you would want to go up one size if you can't find a cheap TXV the same capacity as your unit. That way, when you arrive to a hot home and start the unit, it could "follow the evaporator air temperature up" by opening wide without maxing out its flow.

Some of the rules should be obeyed, though. You want to get a valve the same type as your refrigerant. You want to stick with a high-temperature unit that's not made for a blast freezer or a cryogenic unit. Unless you like making and assembling flare fittings, find a valve with sweat fittings. The discharge side will likely be too large, but that's ok. Sweat some "too big" tubing into the valve and run it close to the evaporator before reducing the size. It will act as an expansion chamber, allowing your refrigerant to vaporize better in the evaporator.