1976 100w superlead

If you like the sound, then go for it!

Don't buy in to all the ckt. bd. versus hand wired tone difference crap.  Any diff. in tone is due to variances in individual component values - not the type of board.  If the tone is good only at high vols., your only choice for getting the tone you like is an attenuator, master volume or a smaller amp.  Not sure a '76 had a MV, but one can be added fairly easily.  Each method has its drawbacks.  You can also pull 2 tubes (both outer or both inner tubes only and adjust the OT impedance), but the vol. change will not be significant.

A 100w SL is loud as hell no matter what you do to it.  Be sure you need that kind of power because if you can't get the tone w/o having the cops show up, it does you no good.  You should save your money and spend it on a lower powered amp. 

One final thought, the tone of a SL comes from the combination of power tube saturation and speakers pushing a lot of air.  You might be able to get power tube saturation at lower vols. with an attenuator, but you can't get the sound of speakers pushing a lot of air w/o moving a lot of air.  That means ear splitting vol. levels - no way around that.

All that said - 500 pounds isn't a bad price if the amp hasn't been butchered.

I have just bought a 1973 handwired, non master volume JMP 50 watt head and it has been a revelation to me.
Until I tried it at the dude's house I had gone through no less than 3 'new' Marshalls - sent em all back after a week or less.
I honestly in all my ignorance thought I 'knew' what Marshall tone was. I didn't... until I played the JMP mk 1.
I bought it, took it to rehersal and was blown away, all the tone I had ever wanted was there in my Les paul and JMP. No effects, no channel switching, no reverb, nothing but you, your guitar and your imagination.
A note on the vol thing, this seems to have just 2 settings... off or really loud! Lol. After 2 on the vol it doesn't seem to get any louder just more gain.
Don't know about the 100 watt version but honestly the difference between old Marshalls and new ones IMO is huge.
They don't make em like they used to. Buy it before they bring out a re-issue and everyone decides they want the original and the price rockets!

PS about the handwired thing, it doesn't make a difference to sound, but i think it's easier to repair a point to point board rather than a fried PCB thats all.

Yep, the best Marshall I've ever played was a '69 Super Tremolo 100.  It had the best Cream era Clapton and Eric Johnson "Cliffs of Dover" sound I've ever heard.  The ONLY drawbacks were it was a little noisy and had some rusty corrosion in places.  I played my Metro 100 Superlead build next to it and they are very close, but perhaps the Metro is a little cleaner and a LOT quieter.  If you ever get the chance try some of the Metro stuff....they are the bees knees when it comes to vintage Marshall replicas IMO.