Generally, 50lb test braid (Berkley Whiplash, Spiderwire, TUFFline or Power Pro are all good braids) with 20lb test fluorocarbon or mono leader.
The 20lb test for your leader is a guide. You are probably looking for a leader diameter of around 0.40mm diameter. With the test poundages of certain line diameters increasing, choose by line diameter and let the test poundage "look after itself". Line diameter of 0.40mm is usually thin enough and invisible enough for all but the most difficult clearwater finesse situations ... and usually gives more than enough strength and resistance to abrasion! (Finesse for me would be 30lb test braid and 0.30mm diameter mono leader!)
I generally think fluorocarbon is a little overrated - it also does not hold a knot as well as mono ... I am currently using DoubleXX High Abrasion in the dull green colour for my leader. Its great and holds a knot well!
For Bass fishing use the Uni-knot to join your braid to the mono (or fluorocarbon). Double the braid line and make 12 turns in the braid - single mono or flouro line with 6 turns. Lubrucate well with saliva before tightening up. This connection has not failed me yet and the resulting knot is small enough to go through the guides and the reel levelwind easily enough.
Leader length for Bass fishing? Anything from 1m - 5m depending upon circumstances and personal preferences. Remember braid does not like rocks and tends to get stuck in wet timber (branches) more easily than mono, so here a slightly longer leader would probably be best. You do not always have to use a leader. Straight braid to lure is probably best when fishing topwaters (frogs) in the slop (soft green vegetation such as lily pads) as the braid will cut stright through the vegetation and not get tangled up ... Dont worry about the Bass being "put off" by the straight braid to lure connection - it does not seem to worry the Bass one bit! And remember, Specimen Carp anglers sometimes use a straight braid to hook conncetions when using boilies or particle baits for Carp with great success. Carp are probably more easily spooked than Bass and the braid does not seem to bother the Carp ...