Yer WELCOME skyspike...
Agree with Bob on most of this, but there's a couple things I'd mod...
If you're talking about an "off the shelf" Sat V v.2.0 then yeah, you're limited to pretty much the RD-180 (two chambers) or RD-170 (four chambers) motors (RD-180 is used on Atlas V-- not sure if we even have the license to get/use/mfg. RD-170's). The other alternative is the AJ-26 used on the Orbital Science Corps. new Taurus II LV. The NK-33 was designed for the Soviet N-1 moon rocket, and it's got the highest thrust/weight of any rocket engine ever built, among its other fine attributes, which is why OSC wanted it for their Taurus II. Problem is NK-33's only produce about 338,000 lbs of thrust at liftoff, so they'd require large clusters, so that pretty much eliminates them for an HLV first stage, just like Merlin 1C/D. That pretty much leaves you with the RD-180/170...
A better solution would probably be to finish the TR-107. At 1.1m lbs thrust, it's a modern-day replacement for F-1. I don't have the specs on the ISP, but it's likely to be better than F-1. A cluster of five of these would be a straight "drop in" replacement for F-1's on a Saturn V type vehicle.
As the "safety numbers" require keeping the separation/staging events and engine count down, I'd go with a monolithic first stage based on the ET diameter, to reuse the same tooling and fixtures to the extent possible and cut costs. Kerosene is much denser and therefore requires a much smaller fuel tank than the hydrogen tank in the ET, so even feeding five large kerolox engines, the first stage itself should be about the size of a shuttle ET. The conical LOX tank would have to go, but that's being planned for in every "shuttle derived" rocket anyway. The LOX tank would be larger, but the kerosene tank would be much smaller than the present ET hydrogen tank. I'd feed the engines with external LOX lines like the present ET uses rather than the "tunnel thru the kerosene tank" ala Saturn S-IC. Using a monolithic stage consisting of a stacked pair of propellant tanks and intertank like the current ET (and the S-IC for that matter) gives sufficiently good mass fraction (which isn't AS important on the first stage as upper stages) while making construction easier/cheaper.
The second stage would be ET diameter and use either a pair of SSME's or a single RS-68. Problem is, either one would require significant development, and I'm not sure an RS-68 can even be airstarted. SSME was found to be 'feasible' to airstart, but would require a development program to accomplish, and that costs a lot of money. J-2X OTOH is already "paid for" and well under way, but it's lower thrust rating means you'll need 4 or 5 to do the job. So basically you'd have to re-create the S-II stage... the S-II was a good stage once it's teething problems were fixed-- of course nobody had ever tried building a common bulkhead tank that size before, which was part of the problem, but it should be fairly straightforward today. Sticking to the 8.4 meter ET size rather than the 10 meter S-IC/S-II size of Saturn V keeps the tooling common. J-2X's are thirsty, so the stage would be quite a bit longer than the S-II, especially when you figure in the smaller diameter...
The third stage would be 8.4 meter ET diameter as well, and would be powered by a cluster of 6 RL-10's. These engines have superior ISP and are light in weight, and would do quite nicely for the EDS TLI burn duties; much better than the lower ISP J-2X. RL-10 is an "off the shelf" and highly reliable engine, one which will undoubtedly be the base for whatever cryogenic in-space propulsion needs arise for landers and such (like the Altair lunar lander descent stage, or the DTAL lander). The CECE engine is an RL-10 variant under development for just such applications, so manrating RL-10 and its variants should be fairly straightforward.
This rocket would be about the same height as Saturn V, perhaps somewhat taller... keeping the EDS stage diameter at 8.4 meters helps keep the height down and make up for the taller second stage. Problem is, this only gets you to about Saturn V performance-- the lunar missions proposed need a lot more payload than a single-launch Saturn V could provide. BUT, going to a dual-launch type system using a pair of these Saturn V v.2.0's would work nicely, and increase the flight rates to amortize the infrastructure costs. There isn't going to be a 'cheap' way to do lunar or Mars missions, so we need to stop pretending there will be (at least in OUR lifetimes). This rocket would also service LEO needs if called upon to do it-- perhaps an "INT-20" version of the first and third stage for simple LEO missions not requiring heavy cargoes, or an "INT-21" version of the first and second stage for lofting 8.4 meter payloads to LEO for Mars vehicle construction if/when needed. A hammerhead to a 10 meter fairing would be easily doable if needed for greater payload volume.
There is something to be said for scrapping the 8.4 meter tooling and switching everything back to 10 meter tooling for all three stages. BUT it would involve a lot of extra costs, but it would also allow for easier upgrading later on, if and when the desire was there to do so.
For cargo-only versions, provisions for adding strap-on boosters, either as Atlas V cores for LRB's or GEMs from the Atlas V or Delta IV would be a good option to have to loft heavier payloads, and wouldn't require a lot of work to "scar" the booster for them in the original design phase.
If SpaceX ever came out with their Merlin 2 engines, these might be a good upgrade, as they're higher thrust IIRC.
There are a lot of proposals floating around... some are quite interesting. I've followed the "DIRECT" proposal for years over on nasaspaceflight.com/forums and been impressed by it, but now I'm seeing more and more the enormous costs of keeping SRB's around, and I think it's a poor trade.
Later! OL JR