Your choice - TT, Arm, Stage, Cartridge - $5000 budget

In terms of going second hand you have a wonderful selection available. 
For a 'big stage' a good phono stage is essential. I had an EAR 834P which has that big stage you are looking for. I own the original Whest phono stage on which I did a review over 10 years ago.
As to turntable... I think one of the widest soundstage you will get is the Michelle Orbe (famous for its wide soundstage) which is a great piece of kit and very arm friendly - a modified rega is often a very good buy. 
don't discount the idea of a good MM cartridge - they give a very bold full bodied sound and place less demands on a phono stage - raulrigas did a very long thread on this topic. I have an old Pickering that I got with a second hand Townshend Elite and was astonished with its' performance - very full bodied. Consider the Denon dl103 as well.

First and foremost, if you are new to vinyl, you need a pro to come to your home and set up your table for you. It should cost between $100-$200 and will be the best investment you can spend as far as turntables go. Having a pro set it up at the pro's shop is second best assuming you don't mess it up when you remount the arm onto the unipivot spike of your Prime, lose the counterweight setting, etc. I own the Prime and have had various Benz and SoundSmith cartridges mounted. I recently had my dealer install the dual pivot ($150) and an Ortofon Quintet Black and it has never sounded better. I have the 3D soundstage now that you have mentioned you are looking for. The Benz's are second best and the SS's a distant third in that category. SS's excel at lively dynamic punchy sound throughout the audio spectrum. The Benz's excel at detail and clarity. The Quintet Black with a well recorded record is the most organic, real-sounding of those that I have tried. Eventually you will want a top phono stage but it can wait. A used Simaudio 310LP or SimAudio LP5.3 or even a new iFi iPhono2 will give you years of enjoyment until you find the dream phono stage you can afford. I went this route (SimAudio Moon LP5.3) and eventually bought a Manley Steelhead that had about 5 hours on it for a very nice price. I've had a ton of phono cables in and out of my system. I had a slight hum problem. I just took a flyer on the Morrow Ph4's with Eichmann plugs because they are well-shielded and despite their veery reasonable price, my hum went down by 80% to a very low level and the SQ is incredible. Whichever phono cables you choose, make sure they are shielded between the Prime and your phono stage and between your phono stage and preamp. 

Trans-Fi Audio tonearm T3Pro, I have two of them.
Audio-Technica AT-ART7 or ART9 cartridge
Moon Sim Audio, used (see post avove)

My London Decca Super Gold does things that I've not heard any other cartridge do. With the optional DeccaPod, it is $1600 new in the US. I would build my analog front end around that particular cartridge; it is that amazing. 

I'm also highly partial to my Fosgate Signature phono stage. They are no longer made, however. If you can find a used one, grab it.

Based on my experience with my Marantz TT-15, I believe that a Clearaudio Concept (same tonearm, w/o cartridge $1400) will suit the Decca just fine.

Although many folks in this situation would spend $4k of the $5K on a turntable, and skimp on the cartridge and preamp, I believe my allocation would result in much better sound.

And, should you wish to spend a little more, I bet that a trip to England to purchase a Michell turntable and a London Decca cartridge could be paid for with the savings on just those two items!

If you haven't decided on a cartridge, consider the Decca.