Bicycle Mechanics - replacing a seven speed drive train?!"

I am a newbie apols for any stupid questions
I am attempting to restore an old orange aluminium mtb. not sure how old it is (if anyone is familiar it has an orange and white frame and currently has maybe original pace race rc35 shox).
I am replacing the drive train and having a brand new xt hg90 cassette I would like to keep it 21 speed. the mechs (xt xtr) are usable i think given a clean but may well have to replace them some time in the future. The shifters are fairly new 21 sp shimano rapid fire, the only ones they still make (stx-rc?) and in fine working order. If i need to replace the mechs what will be compatible?

More importantly I need to replace the crankset and I imagine the bb. The measurementd are 68mm and 35mm will this be compatible with new cranksets (e.g. truvatic and shimano). Are these cranksets compatible with the rest of my drivetrain?

I am also considering replacing the back wheel. As far I can tell the rear dropouts measure 138mm and the wheel measures 130mm. This is somewhat confusing but there is alot of flex in the rear dropouts and i have no reason to believ it isn't original spec. Is this bad news for the aluminum frame? Can I use a 8/9 speed rear wheel (what spacing) and still use the 7sp cassette?

Any advice will be gratefully recieved.
I feel like I've opened pandoras box

I am a newbie apols for any stupid questions
I am attempting to restore an old orange aluminium mtb. not sure how old it is (if anyone is familiar it has an orange and white frame and currently has maybe original pace race rc35 shox).
I am replacing the drive train and having a brand new xt hg90 cassette I would like to keep it 21 speed. the mechs (xt xtr) are usable i think given a clean but may well have to replace them some time in the future. The shifters are fairly new 21 sp shimano rapid fire, the only ones they still make (stx-rc?) and in fine working order. If i need to replace the mechs what will be compatible?

More importantly I need to replace the crankset and I imagine the bb. The measurementd are 68mm and 35mm will this be compatible with new cranksets (e.g. truvatic and shimano). Are these cranksets compatible with the rest of my drivetrain?

I am also considering replacing the back wheel. As far I can tell the rear dropouts measure 138mm and the wheel measures 130mm. This is somewhat confusing but there is alot of flex in the rear dropouts and i have no reason to believ it isn't original spec. Is this bad news for the aluminum frame? Can I use a 8/9 speed rear wheel (what spacing) and still use the 7sp cassette?

Any advice will be gratefully recieved.
I feel like I've opened pandoras boxMeasure the dropouts inside to inside. Current 8/9 mtb hubs are 135,and that is not good business in a 130 aluminum frame. Your BB has a english threaded 68mm shell width. I have no clue what 35mm relates to.New BB spindle length and type has to match new crank. New crankset not an issue with old stuff.

I have measured the inside of the dorpouts :(

nice one on the prompt reply

It's easy to get carried away with an old bike and spend more on it than it's worth.
Before you actually buy anything try to add up the total. It might not be worth it.
You may be close to the price of new bike.

You have opened pandoras box.

Its gonna cost less than 200 hundred pounds to respray, sort out forks, buy new wheel fit cassette buy crankset and service everything else. That wont buy you very much in britain. Infact it rides better than a bike for 250-300 bike even with damaged rim worn chainrings etc. Reckon it'll be worth it :D specially considering I'm gonna do as much of the work as possible.
My main concerns are crankset compatiblity (sydney says not a problem) and the space between the rear drop outs - I've remeasured them and I still reckon they' 138mm and reduce to 130mm when the wheel is tightened on - am concerned that this is suggestive of frame damage? Can I use a 135mm wheel and fit a seven speed cassette with a spacer?
thanks again
alex

Its gonna cost less than 200 hundred pounds to respray, sort out forks, buy new wheel fit cassette buy crankset and service everything else. That wont buy you very much in britain. Infact it rides better than a bike for 250-300 bike even with damaged rim worn chainrings etc. Reckon it'll be worth it :D specially considering I'm gonna do as much of the work as possible.

Glad to see you are already on top of the subject. I hope it works out well. I just wanted to bring it up as It happens where I am frequently.

Sometimes it's worth it to build up a bike, if you have an awesome frame.