Quick Summary:
HP x551 - The most incredible desk-top 'printing press' ever. At under $300. USD it spits out 8.5 x 11" sheets at 4500 SPH. But when it comes to envelopes, it crawls at 700 or so with some crinkling. OK for short runs, but really not a viable envelope choice.
Memjet: Several incarnations of the same $15,000. model available: Rena Mach 5, iJetPress, Colordyne, etc. As with all inkjet output, the color gamut and saturation is less than a toner based machine, so OK for pleasing color but not exact color matches. Even the black is not black...a strong blue-gray. Highly exaggerated output speeds, but it has it's place.
Xante Illumina: Essentially a $18,000. OKI 9650/9800 with some hardware and firmware tweaks. Your best choice for critical color but fraught with consumable (quality) issues, especially the drums. A used OKI 9800 would cost you about $1000. - good way to test the waters.
Xante has a new model built around the new OKI 931, but the drum cost will kill you at about $450. each. You can consider the Ricoh 831 for about $3000. but a $4800. Straightshooter feeder is essential for production.
I presently own all the above in one incarnation or another (and tested the 931) and no one printer meets all my needs.
Good luck.