My storage tank is 1000 gal horizontal. The Tarm supplies directly to top of tank at one end aabout 6" from the top and returns from the tank at the same end about 6" up from the bottom; the tank in effect operates like a large hydraulic separator. The supply has a 24" extension into the tank. Stratification is excellent in the entire tank, supply is at about 12-14 gpm. System draws from top of tank near the middle and returns to bottom of tank by a diptube, also near the middle.
The benefit of stratification on charging is better supply of cool water to the boiler, greater delta-T, and improved efficiency in heat transfer. The benefit of stratification on system supply is realized depending on your need for hot water. If you need 160F+ water, then stratification will be especially important. If you only are supplying low temp radiant, then stratification is not very important at all, as you will need to mix down to radiant supply temperature. I supply only in-floor pex, and I supply at 100F = no need for much stratification at all in supplying the system. In addition, because I mix down, relatively hot water is being returned to the bottom of the tank, which results in mixing.
Maintaining stratification during system supply will depend on flow rate and temp of system return water. If you supply system at 180F and return is 160F, then hot water being returned to the bottom of tank will quite quickly mix the entire tank up towards 160F. On the other hand, if return is 120F, much less up mixing will occur. Similarly, if system return flow is high, more mixing will occur vs low flow system return.
The Garn is a good system, especially where it excels in my opinion: need for relatively moderate maximum supply temperature (approximately 140F or less) and batch burn operation, that is, burn to charge tank, let fire go out, draw from tank to supply system, remove accumulated ashes from Garn to prepare for next burn, repeat, etc. On the physical side, availability of space for the Garn is a consideration. On the hydronics side, the Garn is unpressurized and a heat exchanger likely will be needed to supply a pressurized system, and use of a heat exchanger adds cost plus reduces maximum supply temp to roughly 5-10F less than water temp in the Garn storage. On the chemistry side, the Garn requires annual water testing and treatment as needed. With regard to my statement on need for relatively moderate maximum supply temperature, this is because the Garn will supply from storage from its maximum tank temperature (about 185F) and then draw down to 140F before another burn will be needed, that is, a batch burn operation. Of course, the lower the supply temperature needed, the longer between batch burns. The Garn is not designed to burn more or less continuously because of the need to remove ash, which also may contain a quantity of hot coals, depending on the time between batch burns. So, if supply water needs to be 160F+ or so, the Garn would need to be burned nearly continuously, depending on the btuH of the Garn you would choose, which also relates to the size of the integral Garn storage.
Gasification boiler systems using many of the boilers mentioned by others, with storage, also are good systems. They equally excel in the areas where the Garn excels, as well as better offering more or less a continuous supply of very hot water (approximately 140F and up and also 160F+ water) because most can be burned more or less continuously, if needed. At the same time, in my opinion, these also excel in a batch burn operation, like the Garn, while usually not requiring removal of ash and coals between burns, except on an occasional basis. Due to their small firebox size and design of operation, usually only fine ash and some cool charred coals remain. I empty ash from my Tarm about once other week, and the ash volume is very small.
Most if not all of these other gasification boilers also are pressurized, thus eliminating the need for a heat exchanger to supply the heating system, as well as not requiring water testing and treatment, except on the initial system setup. I haven't touched the water system in my Tarm + 1000 gallons of storage in the last three years. I do take a few drops of water from the system annually to check pH and total alkalinity, which so far hasn't change from setup. Physical install of one of these boilers plus a storage tank may be easier and offer more options on placement.
The volume of storage you will want or need will relate to space available, the btuH output capacity of your boiler, system btuH demand, and how long you want the system to be supplied from storage before the next burn. In my case with the Tarm Solo 40 (140,000 btuH rating) and 1000 gallons of storage, my system typical load is around 12-15,000 btuH average, maximum is about 20,000 btuH. I can heat the storage to 193F maximum, top to bottom, although my usual target is 185F; I can use storage water temp down to as low as 100F, so I can heat solely from storage 185 to 100F before I need to burn again, although I usually will burn when storage drops to about 110-120F. At storage 110F, I have 1000 x 8.34 x 75 = 625,500 btu's available, which at a draw of 14,000 btuH = 45 hours of heat from storage. My typical burn then is every other day, and burn time is about 6 hours, during which the boiler is supplying the system plus charging the storage tank back up to 185F.
If I've been repetitive of others, I apologize. I hope this helps you make a decision.
