I've gained a slight interest in the new version of RuneQuest because there is a Lankhmar sourcebook for it.
How does the new version compare to the original/older version(s)?
I'd want to use the Lankhmar book with the "best version" of RuneQuest.
My personal preference is the 1st/2nd Chaosium editions (which differ on only a handful of details, 2nd being mostly the same text with better layout). Moon Design has published reprints of supplements (including the excellent
Cult Compendium). However, the basic rule book may be hard to find.
The 3rd edition, a Chaosium design published by Avalon Hill, is a bit more complex and features many changes and additions (including a third magic system, sorcery, that often catches flak for being unbalanced). Some people prefer it, and I like some bits better. The saddle-stapled books have flimsy covers, but might have held up well in the box. The (UK only?) trade paperback would probably be most desirable. Failing that, I would go for the "Deluxe" boxed set (not one of the cut down editions, as you'll end up wanting the cut out material).
To judge from the SRD, Mongoose RQ is a pretty odd beast. One reason may be that the trademark (and Gloranthan setting) are licensed from Greg Stafford's Issaries, Inc. -- while the copyright on the old rules still belongs to Chaosium.
The changes to experience seem to me not so good, the new combat round clunky and a bit bizarre. Some folks may like that Hit Points are now only per location, but that some might prefer not to use hit locations in the first place is just one reason to have the "basic" value as well.
The standard rune names are (from what I've seen glancing through books) applied to different actual symbols. The magic system is a mishmash of bits and pieces from the old game plus some new ones. It might work just fine, but it looks like radical change just for the sake of change (and maybe a bit of the "D20 System" style of complication).
The spell list is made up mostly of old Battle (AKA Spirit) Magic spells, but now that's called "Rune Magic". In the old game, Rune (AKA Divine) Magic is quite different. (There are other confusing redefinitions, such as "strike ranks".)
Many rules (e.g., poison) now use a system of opposed % rolls derived from ability scores rather than the old single roll based on the simple resistance formula (50%, +/-5% per point of ability higher/lower).
Hero Points and Legendary Abilities seem like gratuitous additions. The latter mostly have a very "3E D&D Feat" feel. They also include the Rune Lord and Rune Priest statuses, but I don't know how MRQ deals with those. The SRD does not mention Shamans and spirits.
My guess is that it would not be too difficult to use the Lankhmar supplement with Chaosium's RuneQuest (meaning all three previous editions), given the information in the SRD. Going the other way (from old source material to new mechanics) would be harder without familiarity with both editions, especially in terms of magic. The basic characteristics are the same, and a skill % can be used as is or adjusted to taste.
The Mongoose version of Glorantha is set in an age prior to that depicted in earlier versions, which is intriguing. Unfortunately, the firm's track record with editing is none too good and the prices of the books seem to me too high.