Right.
so this is bias towards a nice ish ride at the lowest price without pushing you towards parts I'd say are dangerous. I've excluded changing the forks as those z4 marzocchi's were an ok if heavy fork and pretty bombproof, they certainly aren't modern quality and even something cheap like SR Suntours XCMs would be better. if you were to change them out, I'd suggest you go rigid, but we shall leave those out for now. I'm also assuming you have a little bit of intelligence and do 95% of the work for yourself along with being willing to ask questions when you get stuck or simply don't know.
The challange was sub 100 notes for an acceptable bike so here goes.
parts, with cost and link.
Pedals – Cheapest (or as cheap as I’d go and be happy) £10.99
pedals 1
Bit more but nicer to ride on £14.99
better pedals
Stem (cheap and short and I’d be happy with it, in fact, I am happy with it. Might be a tad short for that frame though) £7.85
cheap but short
other option, bit longer but shorter than the 110mm that’s on there now £10.99
Stem 2, but longer
Bar £12.99
bar
Chain £7.99
chain
Cables brakes 99p each call it 2 quid
cable
Cables Gear £1.99 each so 4 quid (you could save some money and go single speed if you are feeling crazy)
cable
Brake pads £4.99 for a pack of 4.
pads
Cable outer
Brake cable £1.99
cable
Gear £1.49 you should be able to just squish out a full set from this.
cable
Cassette £12.99
cassette
Cant see the rear end but it would be prudent to change the cassette, this needs a special tool, a bike shop should only charge you a couple of quid to fit it for you. I’m making an assumption that it’s a freehub not a freewheel, can’t see the back end to confirm.
then it's a matter of slowly and methodically stripping everything, cleaning it and greasing what you need to grease and tightening what you don't. I'll left out grease and oil etc. from the price so add another tenner there.
so I make that £74.42 for parts. there are some tools that are bike specific you will need, BB tool and crank puller if you need to pull that apart, cassette tool and chain whip to pull the cassette off if the shop wont and a chain spiltter. this isn't the best gear, or even the 50th best gear, but it should get you over the huddle.
bike tools
takes us to 95 quid, so buy yourself a new water bottle too.