Hi Steve,
I find that Sherpa to be a nice looking bike...and agree with all Paul said about it in particular and the Sherpa generally. I owned a lovely Sherpa Mk2 and later rode the equivalent RavenTour on a long loaded tour, kindly loaned me by a friend. Now I own a Nomad Mk2. Having ridden all three, I'd say the Sherpa/RavenTour are good, solid heavy-duty touring bicycles. I found the upper weight limit for good handling in my estimation on rough roads to be about 25kg/56lbs of cargo. I just needed a sturdier machine for my expedition tours where the bulk of my weight is in water and food to sustain my solo back-of-beyond expeditions where I can't readily get to stores for resupply. By far, my biggest weight for desert touring is in water -- 26.5l, consumed at 8.5l/day.
That said, the RavenTour I rode (Sherpa Mk2 equivalent in most respects except for the Rohloff instead of derailleur drivetrain) was absolutely BRILLIANT on the exceptionally poor pavement and dirt tracks of Eastern Europe with four panniers, a HB bag and rear rack-top load (one-person tent and a solar panel there). I had plenty of capacity to carry a couple days' food and as much as 8.5l of water (needed each day in the heatwaves I encountered in Romania and Serbija) and it rode beautifully. I'd call it the ideal bike for my needs in those circumstances, just not for the extremes of expedition loads carried in equally extreme circumstance.
I had a gap in my stable for a derailleur bike with characteristics and weight somewhere between the Sherpa Mk2 and my Nomad Mk2, a sort of bikepacking oriented bike that would still feel lively with fat tires yet was capable of carrying a "heavy" but not extreme load in panniers on pretty poor surfaces. Keying off Paul's good advice, about 6 years ago, I picked up a 2007 Diamondback Transporter that had been a rental bike, then sold off through a reputable pawn shop. Cost me USD$135 and I removed the used drivetrain for use on another project, substituting a full nearly new Shimano Deore drivetrain and some wheels (one with a SON dynamo) I had built-up and had on hand. I prefer drop handlebars and wanted a more compliant fork, so I sourced two Sherpa Mk2 forks from SJS Cycles to get the ride I wanted and bring the handlebars up where I needed. One resulted in neutral handling with 57mm of trail, the other provides a relatively low-trail geometry at 40mm and I enjoyed the handling provided by the latter so much it has stayed in place. I prefer drop handlebars, so I left the steerer uncut and with a short 60-70mm stem extension and compact reach/short drop 'bars, my hands on the brake hoods are within 1mm of where they were on the original riser handlebars with a longer stem extension. It looks a but odd with its tall steerer but works brilliantly and as mentioned, my hands end up where they were before but oriented differently. The top tube looks and is low but mid-tube standover comes at the same place as it does on my Nomad. The crown race-to-dropout measurement on each of the Sherpa Mk2 forks dropped the head tube slightly, making it and the seat tube about 1.3° steeper. It really paid to research it all in advance and this of course affected resulting trail and which fork I sourced from SJS Cycles.
I later fitted some spares from my stock or sourced cheaply...front Tubus Duo and rear Cargo Evo from Craigslist bargains (our version of Gumtree) and bought and fitted a new Thudbuster LT sus-seatpost I got on a stacked sale (factory reduction + store reduction + some coupons and finally store credit...as I recall, it ended up costing me $35 out of pocket). Mudguards came from some rehabbed spares recovered with permission from the LBS trash skip along with new stays from PlanetBike.
The end result is what I call my "Enduro-Allroad" and performs brilliantly in its intended role; the all-'rounder of my fleet and one I find myself riding often because it is so versatile and pleasant. It came with some shallow dents and plenty of scratches thanks to its history as a rental bike, but none affect performance. I'd slot its performance as a tourer a bit above the Sherpa Mk2/RavenTour in terms of cargo capacity thanks to the enormous steel frame tubes, a 35mm top tube, 43mm(!) downtube, 19mm steatstays (same as my Nomad) and the compliance of the Sherpa Mk2 fork and TBLT seatpost. If I were to tour with a front-heavy load, I'd use the fork that provides 40mm of trail; with a rear-heavy or neutral load, I'd go with the 57mm of trail. Ideally, a rear-heavy load would have trail somewhere around 63-67mm. It is as happy with bags strapped to the frame.
Paul makes a very good point about how a seeming bargain often is anything but that once you get it personalized to your needs. If I hadn't already had the needed parts on hand (and a good place to put the ones I took off), the $135 I spent for what amounted to a frame only would have been the narrow end of the wedge toward spending upwards of USD$1,100+ more additional at online prices to get it working to my needs. So...a great project bike for me, basically a frame to hang parts I already had on hand, but something much "nicer" in appearance could have been had if I were spending retail. It also required a heck of a lot of work and thought to get everything working together; in the end I would probably has spent no more time cutting tube stock and brazing up a frame and fork from scratch (I'm a hobbyist framebuilder). The result works great, but it was in no way the integrated solution this Sherpa you spotted is, with all the geometry and basic parts designed to work together from the start. Andy Blance already did the hard work. Provided this Sherpa comes close to your needs, it could be a great bike for you but if it needs some things changed, swapped and upgraded here and there, then it'll get spendy real quick.
Best, Dan.