I purchased the 25l (which is a bit too small) with a view to adding a pump for recirc. The trouble I found was the heater controller is nowhere near accurate enough, it reaches the temperature you set then turns off and falls 3 deg C then turns back on and reaches the temperature you want, then falls 3 deg, etc, etc. So if recirculating your mash, you would have a less consistent temperature than using a lagged mash tun - I lose 1 deg C over a 60 min mash in my cooler box mash tun and lots of lagging.
Also, because of the constant varying temperature and the tall thin nature of it and the sensor being at the very bottom, it makes it tricky (but not impossible) to get the mash to be the temperature you want it to be, even if you are not trying to recirc. Stir lots, have a separate thermometer, etc.
I am back to a 3 tier system, with the Klarstein as the boiler (which you need to set to 103 deg C) but I am setup for recirculating with a pump, plumbing, etc, so may try and replace the Klarstein controller with a PID - either external or built in. With a PID level of accuracy it may be a very different story.
Also, the all-in-one systems with pump come with a separate sheet of metal gauze for the top of the mash and a pipe in the middle to allow water to overflow down into the base, if the water doesn't flow through the mash fast enough.
