The only way you can really know is to do a test run. Make the website live. Get a Google Adwords account running. You dont have to spend a fortune just put a few euros into the adwords account and leave it run until the money runs out.
You can get a counter for you website that will record everthing that hapens on you site..... how many people viewed you site, where thos people are from, how long they stayed looking at you website, what pages of you site they viewd, and a whole lot more like what search engine brought them to you site. You can use the figures from the stats to work out your cash flow projection, and maybe sell a few things at the same time.
You can download these counters by doing a search in Google for website counters.
I only know all this because I had to do it myself, I set upa decking business a few months ago, and most of my work came from my web site which I used Googe Adwords.
Hope this helps and good luck in the new business venture.
Eoin
(The Landscaper)