It's been a while since I had my Venus, but I flew it on YS53 and YS63. It had unlimited vertical even with the 53, so your 50SX will be more than ample power. Mine came with two steel pushrods for the elevators, coupled to one servo. I replaced the steel with carbon fiber, saved a bit of tailweight. Rudder was pull/pull using nyrod instead of cable.

As to the fuel line set-up, I like the short loop outside the fuselage, but I've done that only when the tank is around the CG and I have fuselage space to work with to get the lines out and back in. No question this is the simplest and most foolproof for fueling, de-fueling, running engine dry, pinch test, etc. Also saves the expense of fancy and failure-prone fueling devices. Also no plugs to drop and lose in the grass. I use the Tetra ell fittings from Central Hobbies, also available at my LHS. I guess for a non-CG tank you'd put your tank right behind the firewall, run tubing from tanks out holes in the cowl with a hemostat from your hot air exit area, then do the same for your carb & muffler pressure lines through two more holes, then connect them with brass tubing. Neat idea, think I'll try it sometime. I would put plastic grommets around the holes in the cowl; these are not expensive at Radio Shack.

I set my Venus up with very moderate pattern throws--ailerons & elevators connected from closest hole in to hub on the servo arm and furthest hole out on the control horns. If you do this, you won't need high torque servos. If you're setting it up for 3D, then go the reverse of the above.

I have done throttle servo (Hitec 85) in engine compartment a couple times, and it has worked well. I like to err on the side of nose-heaviness, since that condition is easily corrected with a small amount of tailweight where needed.

As to rx battery, if you're thinking of a pattern setup you won't need anything more than standard servo torque, and you'll do well to avoid the weight of the fifth cell. If 3D, depending on how wild you want to get, the extra torque may be useful. However the Venus is not a heavy model, so I really think 40 in-oz servos on 4.8 volts would be plenty even for 3D.

I do recall that the Venus was my first occasion of CA hinges cracking and breaking (on the ailerons). If I were doing another Venus (and I might) I'd either use Radio South CA hinges (the best IMHO) or conventional hinges (Dubro, GP, Hayes). For 3D, I think I'd definitely use conventional hinges.