even if you didnt improve the building's heat retention, 40kW still seems high. your potential source of fuel is very affordable, but unless you plan to operate a year round B&B and jacuzzi, I think your overall ROI is not good let alone your carbon footprint. i doubt 6k factors in your time to forming the tappings on the cylinder and getting the insulation right, installation and commissioning time. in terms of operation costs, the standing losses on 2250lts will be very high and a horizontal accumulator could be very difficult to operate efficiently. maintenance costs/time will depend on the quality of your timber.
if the combined building's form and orientation are right, i would focus on modifying the structure to avail of solar radiation and natural daylight where possible.
with regard to improving the wall's insulation levels, there are two feasible options.
1. internal insulation on external walls only.
dry line with pre-insulated slab & mushroom nail or slab/insulation/batten arrangement. generally, 60-70 mm of Polyisocyanurate (polyiso or PIR) brings you very close to current building regs. In Ireland, grant support of up to 2k is available for this type of energy improvement measure.
-works well and cost effective if you qualify for grant support.
-very disruptive (depending on your programme) and minor loss of floor space.
2. external insulation
since you probably have no scaffolding costs, this could work out at a reasonable price. shop around as prices vary widely, at least over here.
-very effective and no disruption to any activities indoors.
-can be costly, labour intensive and additional supervision is recommended.
if you plan to spend alot of time in the workshop, consider a passive solar design by using roof windows on the southern aspect. this will minimise heating costs also provide task lighting at no additional cost. keylite provide very affordable roof light systems.
supplementary heating could be provided by the main central heating
and controlled via a zone valve and room thermostat. alternatively, you could consider a gas fired heater or standalone solid fuel stove with exposed flue.
some links on dry lining systems:
Quinn
http://www.quinn-therm.com/wallinsulation/masonry-laminate.cfm
Kingspan
http://www.kingspaninsulation.ie/Products/Therma/Therma-TW56/Overview.aspx
Extratherm
http://www.xtratherm.com/products/thinrthermallinermf.php