There are so many variables here, it's hard to know where to begin. I think the fact that most of the original interior is now junk due to the sawzall/hammer/prybar technique, seriously negates your ability to get a substantial amount of money for the AS in the future, should you have to sell it...unless you have some mad skills, and can turn the thing into a show-piece. Even then, you're talking big bucks. And the potential buyer for that type of trailer is in short supply (imho). People pay more money for an original vintage interior, than they do for a rehab job. If they are going to spend that kind of money, they may as well buy new....or newer that doesn't need a lot of work.
Solar is of limited value to someone who wants to take the camper to State and Federal campgrounds...due to heavily wooded areas. If they are going to park it in the desert sun...maybe. But who would want to live in an aluminum oven? The solar will not run your a/c, nor will your batteries.
These days, you could do without the gas oven. There are electro-mag cooktops that generate very little heat. There are microwaves and toaster ovens....and cooking outdoors over a campfire. Of course, none of these will run off your solar or batteries either...for very long. You will need to be plugged in to power most appliances. Sure, a BIG solar array might mitigate this issue...but the technology is not yet there.
Definitely NO NO NO to outdoor speakers. Nobody in a campground wants to hear what you are playing. And these days, with bluetooth technology, there really is no reason to have a wired stereo in your AS. Besides...wired stereos are phantom power draws that will drain your batteries, unless you have a bypass switch...but then you have to keep re-saving your stations and settings every time you power up.
You could live without tanks. There are portable black and gray water tanks. But you have to empty them. And that would seriously affect your resale value to anyone who wants to actually use the camper as it was intended.
There are people who can live without a propane fridge...because they believe they will be plugged into 30amp service most of the time...and can get by when they are not. But the standard for TTs is propane/electric fridge...so that you have the option of either/both and keeping your food cold while traveling.
I believe I can get by without a toilet and shower, because most of the time, I will be in established campgrounds with their own showers and toilets. Let someone else clean up the mess, I say. But other people...potential buyers of your unit...who have children and want to travel...may feel a shower and toilet are mandatory. I installed an outdoor shower on mine....so when I am boon docking...I can just step out the door and shower under the stars.
Storage is always a problem in ASs. You cannot plan to store a lot of gear and supplies behind the wheels. The frame is not strong enough to carry a heavy load while bouncing down the road. That contributes to frame failure. So, you make do with what you can. But there is only so much room in an aluminum can (I say that affectionately, of course).
Most people rehab vintage Airstreams for the love of it....not for potential resale. If you do the job right, it will take a long time...and cost a lot of money. But you do it for the love of the Airstream...and the Airstream life (Ok, so that's a bit overly romantic....). No matter what you do to the Airstream at this point, many of your 'potential' buyers will not like it....and won't pay you what you think it is worth.
If you are not into the Airstream for the love and iconology that it represents, sell it now for whatever you can get out of it. Give someone else the chance to pour their love into it for themselves and their family. And get yourself an SOB that will suit your purposes.