Painting a summer house

I've been asked to re-paint the outside of a summerhouse with Cuprinol shades in cream. I know I need to key the surface with 120g paper, where there's rot they've asked me to use a wood hardener and fill with Ronseal 2-part filler, then paint/make good when sanded and dry. This I can do.

On the inside, it is nearly 4cm thick T&G, The previous owner spent £17k on the summerhouse. I've never seen T&G so thick but as it's bare wood it has got filthy and is showing its age. This is my plan, please tell me if I am doing it wrong.
I am going to 80g everything, then 120g with my Mirka to get it back to clean wood but they want to keep the wood grain look. Now the bit I am stuck on. Would an oil keep it nice? It is too big an area to wax at 18ft x 12 feet and with there being internal partitions there's more wood on the inside than the out. My worry is if they want Cuprinol out and oil inside, will it trap moisture in the wood and cause rot, or am I overthinking it? I suggested OSMO but don't know which one to use internally that would keep the wood looking as it is as they love the look, so nothing opaque. Their intent is a gym/workout room, a boxing bag on the wall and general summer room when the weather's good 'up North.

Sorry, long post but thanks

 

I don't know what is currently on it Astramax, it was painted before they bought it but it is cream in colour, not that that helps.

They've already bought the Cuprinol cream paint and the white in Cuprinol for the guttering facias etc and we used it a little the other day and it looks ok'ish after 3 coats but was like water. It was running down the brush handle as we were painting the underside of the roof with the white too, it got everywhere. It needed the 3 just to cover it from bare wood.

 

Thank you very much