I'd say the market is small - it is a niche product that is expensive and most users perceive phase to be a daunting and potentially dangerous experience (bad insulation = dead hardware, quite complex to install/setup).
You need to gain a reputation which will take time, to build up a customer base, you also need to be prepared for your money to be tied up in assests until you start turning a profit - which might not ever happen.
You are going to be competing in a market that is already dominated by bigger players, taking a slice of a small pie won't be easy.
Not easy but possible.
Water is becoming more popular but far from 'normal'. Normal is getting a pc and leaving the stock HSF on! Water cooling is 'normal' amongst enthusiasts, but you need to maintain a perspective. On sites such as this we are all enthusiasts that are exposed to people who make what we do look tame, air cooling is tame compared to water, water to tec, tec to phase, pahse to ln2... Attempting to put a figure on this, it would be fair to say approx 90% of the market use air to cool. We (enthusiasts) are a sub division of that 10% and of that a percentage of us water cool (not everyone on here uses water - maybe do a poll 'what cooling do you use' air, water, tec, phase etc to get some figures of the market, it may be worth adding what country you are in and if you would be prepared to use more extreme cooling at what cost. Perhaps join xs and post something similar. The more of the market you can assess the more realistic your expectations will be.) a smaller percentage are prepared to go further and use tec/phase...
I admire the fact you want to do this, I've thrown in my opinions for what they are worth.
My advice (hope you don't mind, I'm certainly not claiming to be an expert here, just offering up some ideas) would be to carry out market research and then scruitinise those results, if 10 people say they are prepared to do phase, I imagine only 3 would - converting potential sales into actual sales is not easy.
Best of British my man!
Buff makes a good point, you need that Unique Selling Point...