headliner is actually pretty easy in comparison to outer hood, just has a few strips of fabric hooped over some bars in the roof and secured with contact adhesive. I think you might be able to manage with hood still on, though if the insulation pad is damaged it will likely just fall to bits inside the car which would be bad

I've made a new insulation bag as well, that bit is quite time consuming so unless you can find off the shelf its best to leave a couple days to do it. plus obv need to have somewhere undercover to store the car!
dont forget 3 new tension cables sides, and be aware the side ones changed over the years so verify waht you have.
I did a complete hood refurb over 2 days, that included an unforseen change of hood frame due to lots of hidden rust and a headliner swap as the one on the replacement frame was shafted! I was lucky I had a complete spare hood frame 'in stock' otherwise I'd have been a bit buggered!
If I were you, I'd get on the MK1 owners club and keep your eyes on the for sale section, you often see complete good hoods & frames going. A complete hood swap is pretty damn easy, only a few bolts hold it to the car at the back and you just have to tap the hood into the slot around the back of the car. Days work tops! The MK1OC also has an excellent hood replacement guide in PDF format, handy to have even if you only end up swappin a complete frame over