I’m assuming yoru trying to build an AM/Enduro type bike here so sorry to reply all weight-weenie… but light bikes tend to feel better on the ups and downs, and hence if your specifying from scratch why not build light and strong?
EX721’s are very strong but very heavy… if you don’t need world cup downhill strength because your not nuts* (or clumsy) you could probably get away with a lighter rim. Stans Flow are the obvious choice as they are wide which gives the tyres a good shape, and yet almost XC type weight. Not the greatest when you do dunt them as there is not a lot of material there but a good compromise for weight vs strength. A Flow will probably need replaced more often than an EX721 but will feel noticably lighter. There are some Chinese made carbon rims which are similar to a Flow rim available if you fancy a gamble?
Hope stem and Renthal bars is good kit but why not go Carbon bars and a lighter stem if your buying new? Easton 35mm Havoc/Haven are the obvious choice and would be lighter – others are avialable.
Reverb – obviously! I tend to buy cheap saddles as they are just as good and light as expensive these days. Madison Flux/Charge spoon if they fit your arse.
Float forks vs Marz RC3 Ti… I’ve got both. The Floats are 150g lighter but don’t feel as good. Does not feeling quite as good matter when they are actually both very bloody good?
You can say the same for rear shocks. I have a custom tuned Float R shock and a CCDB with a Ti spring. The CCDB does feel better but again the benefit is not huge and the Float is perfectly happy doing all the same stuff… in fact for some riding the Float combo feels better as they give more pop and feel more playful, it’s only in the scary gnar and at great speed that the CCDB and RC3 Ti makes it all much smoother and are worth the (quite substantial) weight penalty. I’m glad I’ve got both but if I was to only have the one option it would be the Floats.
Brakes – I’m a Formula fan so TheOne’s would be my choice.
Gears – if you can get away with 1×10 then I’d go XT or XTR shifters with a Zee or Saint rear mech. If I needed more gears I’d ignore the weight penalty and fit a Hammerschmidt.
No point trying to save weight elsewhere and then fitting heavy DH tyres so look for a compromise tyre… but as with the rims you might end up trashing a few to find out what works for your riding. Conti rubber queens and Scwalbe Hans Damf are good and much lighter than a full DH tyre. Perhaps only fit a DH or UST tyre on the back?
* if your a nutter who breaks bikes then check the warranty and history of Alpine failures carefully… they are not a bulletproof frame option!