Well it depends on your budget and if you want to future proof things to what system you get they all have their pro's and con's. If it was me i would go for a 40/40 or 40/21 system. You can also get the cheaper 300gpd systems any of the above are going to make enough water for you.
Ive had both in the past 300gpd system and now currently a 40/40 system and the 40/40 is far better in my opinion and is the common one most people probably use.
Pro's for the 40/40 or 40/21 (all the numbers reflect the size of the membrane 40/40 is a 40 inch membrane 40/21 21 inch membrane)
They are easy to setup you have 1 pipe going in at 1 end and 2 at the other (1 for the pure and 1 for the waste water) all half inch hose.
They are very efficient and can produce water quickly
Its easier to narrow down any problems you might come across then the 300gpd ones
Cons, they are more expensive to buy and the membranes cost more to replace (although maintained properly it should last you years for the amount of water you want to use)
The 350gpd ones are pretty cheap and will still still make enough water for you but they are not as efficient and ive just found in the past with the smaller 1/4 inch tubing and the need for 3 membranes connected together if you have issues its sometimes harder to narrow any faults down.
Also because your tds is high the 40/40's tend to get your tds lower then a 300gpd before it goes into your di resin (so saving on resin over the year).
For example in my old lock up my tds was about 385ppm and the 300gpd r/o got the tds down to roughly about 25-30ppm before going into the di resin, in my new lock up my tds is 560-600 and my 40/40 gets the tds down to 17ppm before di vessel.
if you go here
http://www.daqua.co.uk/ro_systems.htm you will see all 3 systems mentioned above and what they roughly cost.
Also take into account if you're on a water meter, with your high tds you will need to set your pure to waste ratio to around 65% -70% waste to 30-35% pure so basically every 100lt of mains water going into your membrane you will only make about 30-35lt of pure the rest goes down the drain. You could set it to say 50/50 but that just means you're going to work your membrane harder so it wont last as long, so you need to see which way is more cost effective for you.
All those systems come with the 2 pre filters you will need before the water goes into your membrane (1 sediment and 1 carbon filter) these are to protect your membrane. The sediment is self explanatory it catches any bits of sediment the carbon one removes the chlorine etc in the water that can kill the membrane. Both need changing from time to time depending on how much mains water goes through them but there not that expensive (but important). Personally i have the sediment filter in a clear housing as its easy then to see when it needs changing without having to take the filter out (as it starts turning green).
Once you've decided which one your edging towards best to ring them up and they will be really helpfull.
there is this place ( very helpful guy and wont sell you want you dont need)
http://www.daqua.co.uk/ro_systems.htmA company called gaps water same as above very helpful and know there stuff too. Personally i find daqua slightly cheaper.
It would also be best to mention your pressure and flow rate to them as im not sure if you may need a booster pump your pressure is good (55psi) and probably suitable but r/o membranes work best at about 80psi.
You will also need a d/i resin bottle to bring your tds to 0 (this goes in between your holding tank and the pure outlet on your membrane and a bag of DI resin (an 11lt vessel would be ok for you, ideally to help the resin last longer its better to have 2 but not a necessity and dont worry about that right now).
Just to add you could also use any of these systems set up in your van too.
Hope that wasn't to confusing, any questions fire away (the above is just my opinion and findings from experience)