I've been using Lee presses and (only) Lee dies for 20+ years, and so can't comment on the RCBS/Lee interoperability. I have both a Turret (~20 years old, 3-hole) and Loadmaster (5-hole) press, plus a couple other Lee single stage presses that I use from time to time for special purposes. Lee's expander dies are made to be used either as plain expanders, in which case you can also pour powder down through the central opening when the cartridge case is up in the die, or with a Lee powder measure screwed on top (after you remove an upper collar on the die, which retains the expander stem.) I've always used mine with powder measures installed, and they have always proven to be quite reliable. The measures are cheap enough that I just leave one installed on each turret.
At Midway, the Expander Die alone for 45 ACP is $8.29, while the full 4-die Lee set (includes the separate Factory Crimp Die) is $26.89. I find it much easier to seat and crimp in separate stages, and the Lee Crimp Die is generally regarded as the Cadillac of dies of this type. If you have a 4-hole turret you can crimp separately too. You might not care about separate crimping, and if so you could save a little money with the 3-die set for $19.99 (no separate crimp die.) In either case you could sell the RCBS dies that you have to offset the cost of the full die set.
If it were me, I'd just go for 4-die sets in the calibers I wanted to load, and sell the existing dies to cover part of the cost. Least cash out of pocket, of course, is just a new expander die for each caliber.
In any case, consider getting the update kit ($13.99) for at least one of your Lee powder measures. It offers 4 advantages: it uses thumbscrews to attach the hopper to the measuring base (you have to remove the hopper to change disks) instead of sheet metal screws that need a screwdriver; the hopper can be twisted to shut off the flow of powder (occasionally convenient); there is a swivel adapter that lets you orient the measure in any direction independent of the rotation of the expander die body itself (get the expander set to the right depth for the amount of expansion you want, then orient the powder measure independently so its operating arm isn't running into any of the other dies); AND it includes the micrometer adjustable measuring bar ($10 by itself) which is continuously adjustable instead of using the fixed-size disk openings. I've upgraded all of my Lee powder measures with these kits, and consider it money well spent.
Do NOT get the Pro model powder measures, which come with the swivel and round hopper. Those don't come with the micrometer adjustable bar, but more importantly they are made for use on the Pro 1000 and Loadmaster presses which use a chain to return the measuring bar to its starting point to pick up the next load of powder. On a turret press you need the spring return mechanism, which comes on the standard disk powder measure.
Enjoy the turret press. I always found mine to be a very functional tool. It is vital that you follow the setup instructions carefully, and don't be surprised if it takes a little fiddling to get the thing running smoothly the first time. (There are some extensive FAQs on the Lee web site, and you can talk to real people at Lee if need be.) Once it's up and running it will stay running if you keep it clean and keep the turret edges lightly greased. At some point you probably will want to pick up a couple of the little nylon clutches that turn the operating rod in the center of the turret. That's about the only wearing part on the machine. I think I've gone through 3 of 'em in the 20 years I've had the press, but you'll want to have at least one on hand, just in case.
