I started with a Ro-Man 100gpd unit,.. it actually produced only about 200 litres a day,.. maybe a little more in warm weather. I had it running 24 hours a day, normally 5 or 6 days a week filling a 1000 litre tank in my shed. It kept up with my needs until the xmas rush,.. I needed to get 1 fill from my dad's tank to tide me over,.. but other than that it did the job. Now I use a Merlin which is 720 GPD, and its only switched on 2 or 3 times a week for no more than 12 hours at a time,.. but it's very handy to have the extra capacity if i need it,.. If I have a helper for a week for example.
Do what Tosh says and set it up filling water buts or an IBC or something and leave it running 24/7. Adding an extra membrane will double the output for £50 or so. 6 or 8 months from now when you are earning, then invest in something more substantial.