150-200l seems a bit small to me?. I have a 500l tank in my connect lwb 230, and while i have thought i could get away with downsizing, i certainly wouldn't go for anything smaller than 350l.
But, look up watertanksdirect.co.uk, they have a huge range of tanks for all different uses. They do a 210l tank which may be suitable for what you want, its dimensions are 420mm x 630mm x 920mm, and that can be had as a flat, upright, or tower.