I really don't see an issue with the rooflines. Looks like the angles are fairly similar and the clean lines of both structures should be easy to coordinate, especially in your open, rural setting. Since you've found a stunning deal, I would say, go for it! I definitely would want to find an architect ASAP and by all means, before you pour the slab, make sure you have a plan to coordinate.

Clear any shrubs / trees / weeds, away from around the house - not saying go out with a chainsaw and slash everything down to the ground but make sure there is nothing higher than the window frames directly around the house or even leaning on the house. Taller tree's may be able to be pruned to allow more light through e.g. lift the bottom of the growth up higher by removing lower branches or thin out branches from within the canopy. Bring in as much sun as you can during the day. If you are planting a new garden, always look at the height of the tree's when they mature and if possible mix some trees that lose their leaves in winter so you obtain maximum sun. Check your insulation e.g. last year I added a new layer of insulation onto of my existing ceiling insulation. If you have dampness under your house maybe look at channelling water away from the house with what ever means you can afford e.g. professional drains right around the house or just dig trenches. You may wish / need to add black plastic over any clay / ground under your house if you are not on a flat concrete block based house. Open curtains during the day. Dress to the conditions. If you can't afford to heat the entire house just heat the room(s) you are using but make sure the non heated rooms get ventilation from time to time (especially lovely sunny days). Pile the blankets on the bed and even use a blanket over your legs if you are sitting watching TV or working on your computer. Think SUN SUN SUN. If you are renting and the house you are in gets very little sun, now is the time to start looking for something that gets more sun. Winter is the best time to look.

I have no idea what the relevant building rules are for your area , but I like your 'jailbar' idea , timber ( I'm guessing you'd need to get good solid timber -- no knots that may crack when someone runs into it playing indoor rugby haha ) stained dark -- it would contrast and compliment the lighter honey stained timber . It looks like your idea though is to take it straight across to the window ? I'd follow the existing L shape -- it would make the room a bit darker , but the stairwell would be as light as at present , plus there'd be no extra problems with taller people or furniture up the stairwell .

I'm renovating my garage and under inspection on the Building Standards the 2.4 meter minimum requirement is only for new homes, while 2.1 is the minimum for existing homes. Anyone have any clarification for this?