G'day Allen, here's my opinion—interspersed with some facts.
My first ARF was an early Oriental. After getting that model, I resolved to only buy ARCs in future because I was worried about sealing all the joints in the covering.
I have two ARC Vectors that I am working on from time to time—when the mood takes me. With my flying skills, there is no rush.
Engines—from my reading, others say the Brodak 40 is fine. For size, the Brodak 40 is a little shorter than the FP and the extra length of the FP means that you need a spinner with a backplate that wraps around the prop driver—like the Dubro item. That said, plenty of people are happy with an FP 40 and several years ago, Novice at the New South Wales State Championships was won with an ARF Vector powered by an LA 46.
I don't like the crimped tubes on the leadouts and the braided cable leadouts run direct on the nylon bellcrank. I've taken the bellcranks out of mine and replaced the leadouts with swaged terminations and the cable now runs through the bellcrank on pieces of tubing for bearings. I've also taken the opportunity to reverse the bellcranks so that the front line is up. Does it make a difference? I don't know, but plenty of championship winners tell me that it does. To remove the bellcrank and reverse it requires two holes in the centre sheeting on the underside of the wing—one to get to the existing bellcrank mounting screw, the other to permit drilling the bellcrank mount for the reversed bellcrank and to refit the bellcrank. I was able to cut the holes using a 1/2" dia gimlet pointed dowelling drill, turned backwards by hand. Mine are ARC versions so threading the leadout cables through the wing is easy. I have not replaced the leadouts on an ARF model but others have and should be able to explain the process.
You also need to check the leadout positions in the adjustable guide. One of my two had the leadouts reversed at the wing tip so the cables were rubbing on each other inside the wing at all times.
Everybody in my club has reinforced the landing gear mount with pieces of 1/4" square or triangular material to spread the pull-out load of a heaving landing into the fuselage sides a bit more.
Also, we have had a couple of Vector 40s break their backs at the back of the wing cutout after heavy landings. There is a failry large cutout in the bottom of the fuselage to allow the wing to fit and there is a filler piece to complete the fuselage. The fit of this part is not all that flash on mine so I will be taking steps to maintain the lower fuselage line and fill any gaps. However, to prevent the models breaking their back—and this has been done with a couple of Stregas as well—cut slots in the bulkheads at the front and back of the wing opening on the fuselage and the fuselage filler to accept 1/16" ply tongues. The wing is then glued in and the under-wing fuselage piece is glued to the fuselage sides with the ply tongues as well as just gluing the piece front and back. None of the models modified this way have failed in a heavy landing.
I'm happy with the front pushrod for a model of this type but mine have clevises at both ends of the flap–elevator pushrod and the clevis pins are a rattling good fit in the holes in the horns. I would bush the horns to remove all slop from those connections. There are plenty of people who will tell you not to use clevises—not negotiable. I don't disagree with them but I would say never, ever run clevises with pins of 1.5mm or 1/16" dia rattling in holes around 3/32" dia.
If you opt to replace the control system with ball links, replace the horns with others designed for ball links. I replaced the clevis on the front of the elevator pushrod of my Oriental—there was no room for a ball link at the back—and the front pushrod binds on the ball link limiting the amount of up elevator. The up elevator on the Oriental is adequate but I would like a bit more. The binding did not show itself until I had glued everything in. If you want to use ball links, Tom Morris can supply a flap horn to suit.
The cowls supplied today are fairly flimsy fibreglass mouldings which I'm replacing with conventional balsa cowls. I have a die-cut kit as well and the cowl sides were fairly badly crunched so I'm building three cowls simultaneously. It makes such mods fairly easy.
There are a few other cosmetic changes that you could make if you elect to go with an ARC model to set it apart from everybody else's ARC/ARF kits but that's up to you.
Flying Models October 2011 has a rather good article on building an ARC Vector where the nose was shortened to allow for the extra weight of an LA 46. Apart form the nose shortening, there is a lot of useful information about the build in the article so it's worth chasing down.