Nick Hoffman says:
"I recognise the numbers are pretty rubbery, but that looks like $10-15 billion to me. That might be "cheap" in your book, but for that I suspect we could build a new heavy lift system, or at least build a docking adapter for the Energiya and get it all done quicker, cheaper, and safer."
Tom Brosz says:
"If you're talking about dumping billions of dollars on this, I'll bet the Russians could get the Energia booster program back on line for a hell of a lot less, and that beast will launch just about anything."
Wade says:
"The Energia-Buran Reusable Space System (MKS) was developed as a system that would duplicate the capabilities of the US shuttle system. This decision was taken on 12 February 1976 by the Soviet leadership (Brezhnev, Ustinov, Keldysh) following the loss of the moon race to America. This had pointed to serious deficiencies in the technology base of the Soviet Union. The time-honoured Soviet method of rectifying such situations was to copy the foreign technology. The Buran decision was contrary to the opinions of the Soviet Chief Designers, who favoured smaller reusable ballistic capsules or spaceplanes, and the Soviet military, which preferred a new family of modular, lower-tech, expendable launch vehicles.

Following extended development, Buran made a remarkably successful first unmanned flight on 15 November 1988, four years late to schedule. But the it would never fly again. The Soviet Union was crumbling, and the ambitious plans to build an orbiting defence shield, to renew the ozone layer, dispose of nuclear waste, illuminate polar cities, colonise the moon and Mars, were not to be. Funding dried up and the Buran program completely disappeared from the government's budget after 1993. "
http://www.friends-partners.org/mwade/lvfam/energia.htm

I say if you NEED to deal with the Russians, you better keep it pretty simple. Restarting Energia booster program is not what I would call simple. The main problem with getting into space is not technical- it's always been political. The politics involved with re-starting Energia would be formidable. Where is the money neccesary coming from?
Not NASA.
The private sector is technically capable of building Energia or Saturn V type boosters- there is simply not enough market demand.
If there is a need for a large Saturn V type booster, one could be built- either with completely new design or you could modify exists designs.
Shuttle C could be fairly easy- if you were happy with about 80 tons to LEO. If you NEED 100+ tons, it would probably be better to start from new design.
Now if I was king, I would use my pipelauncher and the Russian Proton- this makes going to the Moon pretty cheap, but wouldn't be nearly enough payload size for a Mars Direct:).
-gb (Mars Direct sucks, anyhow:)