For a tank that large I dont know an exact poundage suggestion, but usually the more the merrier for live rock. If it were my tank I'd spend a bit on base rock (cheap live rock, or some dead) and then spend a large chunk of money buying some large show pieces of live rock...they can be very hard to find, but if you just get a bunch of little cheap dense rock its going to be pretty hard to aquascape the tank the way you want to. (although a lot can be said for using zip ties and other ways of connecting the rock to each other)
For a 125 you have SO many possibilities of fish that you could add...I'd suggest taking a look at some sites that have a lot of options like
www.liveaquaria.com to see if any fish strike your fancy...check out your local stores and do the same. If you find one or two fish that you REALLY want, you can kind of design the tank around them and make sure they have exactly what they need...and then maybe add another few fish that would match well. Since you have quite awhile to decide which fish to put in while your tank cycles with the LR and you get everything right...there's no rush to figure out your exact stock. A lot of times the way that people run it is to get the plumbing and equipment right (tank, lights, heaters, powerheads, skimmers, sump etc), get the hang of water chemistry (mixing and aging water for water changes), add Live rock to cycle for a couple weeks or a month...add a cleanup crew (crabs, snails etc), then add fish SLOWLY.
Would love to see pictures...thats a nice big tank