As you point out, stratification is significantly better with tall, thin tanks than short, wide tanks.

If you go for a short, wide tank you could end up with all the water in the tank at close to the same temperature, meaning it will be hard/impossible to retain high temperature water for DHW and use lower temperature water for your heating system (if any). Also, getting the whole 2000l up to the temperature needed for good DHW is going to take a lot of energy. A large tank of tepid water isn't very useful.

Also, if you're using a tank not intended for hot water storage make sure you factor in how you're going to insulate it. With a tanks that size you should be looking at the equivalent of 100mm of good foam insulation.

I think it would most likely work OK, but as MarkBennett points out good insulation is of paramount importance. What are you going to do for DHW supply? With only four 35 mm connections, presumably two for boiler flow and return, and two for CH. you could ideally do with another two, for example to a plate heat exchanger. The main boiler flows and returns usually are bigger, say 42mm or more and they would need to be diagonally opposed. If the system is to be pressurised, be careful too about the quality and the metal thickness. If you're using an old tank a pressure test would be advisable.

What is the space dimensions height width and length there is a tertiary store that may fit I deal with and it is highly insulated, why do you need stratification if it is only for biomass, stratification is more important for heat sources such as solar, so yes it will work for you.

The Laddomat fine, but the same can be acheived by using a heatloss radiator on a gravity circuit, thermostats to operate a pumped circuit via plate heatexchanger (HE) to heat your store and then the return from the plate HE can be injected back to the flow to the gravity feed on the heat loss radiator. Controlled by thermostats you would achieve the same thing the Laddomat does in essence. This will however keep the store indirect to the stove, so the store circuit can be pressurised if you wished it to be, the heat input to the store coming from the plate HE can be controlled by the return on stove side of the plate. This will ensure heat is only taken to the store when the stove circuit is fully up to temperature and will regulate itself. If the return temp starts to fall the store side pump will switch off and the heatloss across the plate will fall maintaining the store circuit temperature, also if the return back to the stove falls then the stove pumped circuit will switch off leaving the stove gravitating until the stove gets back to temperature.

The Plate HE if sized correctly will dissipate the heat but when the store reaches temperature then that is why you have a heat loss radiator which you would have to fit also with the Laddomat it is a requirement that a gravity circuit 1/3rd of the stove output must be accommodated by a heat loss radiator under UK Regs.

It depends on the boiler I admit I missed that this is a log boiler in the question posted by the OP, but it will depend on the log boiler if it is tested for UK and the safety features accepted because the regulations for the UK on solid fuel appliances require a heatloss circuit. There are systems out there that have cold water flow dump valves to quickly dissipate heat from the boiler HE via a cooling coil inside the HE not all are approved for the UK.

The cost of the Laddomat is £360 list? two pumps and plate heatexchanger two limit stats wouldn't be dissimilar so cost really isn't an issue. The Plate heat exchanger means that the cylinder and heating circuit can be connected as a sealed pressurised system if required rather than open vented and back up heating from a sealed pressurised Gas or Oil boiler for example can be incorporated.

It really depends on the system as to which is more applicable if it is a log burner and a store for heating then either there is a coil in the store to produce hot water or the hot water is generated via a plate heatexchanger from the store or the store feeds a secondary hotwater tank. The laddomat is perfect for open vented systems such as that but it isn't the only option.

Have you looked at http://www.akvaterm.fi/eng/Accumulators/Akvantti.40.html
It is 1840mm high with guide pipes out of the side of the tank.
We usually work on a ratio of 50:1 on litres of thermal store to kW output on our wood gasification log boilers.
Therefore i presume you are looking at a 40kW.
We have had customers use the AKVANTTI1400 (1660mm High) on our 40kW boilers but AKVANTTI 2000 preferable.
They have very low heat loss compared to other thermal stores due to seamless polyurethane insulation rather than foam jacket.