As most peoples guests don't seem to be dirty/sweaty I'm assuming Mr TIM will have friends around tinkering with cars in his workshop so Mr TIM will be getting dirty/sweaty out there as well.
You have the bathroom downstairs so let guests know they can bring a change of clothes & take a shower, keep a pile of guest towels (maybe not white) so Mr TIM can give them one when they have finished working & suggest they have a shower & clean up before you have lunch/dinner or whatever. If Mr TIM takes a shower after working outside then guests who may be reluctant to may eventually take the hint, you could even make a comment about having enough bathrooms so they can bring a change of clothes next time & freshen up
If in & out of the workshop/garden or you have children visiting keep the 'mess' contained to the alfresco & family/kitchen/meals areas, a leather sofa gives here gives you the easy clean you want. Also if this is the area you concentrate on furnishing first guests will be used to casual socialising here & dirty/sweaty guests are unlikely to suddenly go & sit in the formal areas once you furnish that area, unless invited to. Your casual area is a lovely space so I doubt any guests will feel they are being banished to the back room because they are not clean.
As you are furnishing 3 living areas I think you can stick to your guns & get a fabric sofa in the front living room.
I will also put in a vote for macro suede, mine are a darker colour & so easy to keep clean.