Based on your printing needs, I would recommend you get a R2880 to have a good quality Piezography printer that you can easily maintain and store with PiezoFlush when sitting unused. The R2880 uses cartridges that install directly into the print head, so you can easily change cartridges if you wish to print both Digital Negatives and regular Piezography K7, and install a set of flush carts to flush the print head for safe storage when the printer won't be used for a while.
If you get a pro model printer with internal ink lines and dampers, such as the 3880 or 4900, you will not be able to change inks or flush the printer as easily, and will waste a lot of ink each time you want flush/store the printer or resume printing. Regular use is good to keep any printer working well, but I'd say even more important with a pro model printer, because lack of use can cause issues in the lines and dampers, as well as the print head.
The 4900/7900/9900 models are fantastic because with 11 cartridge slots, you can print both matte and glossy with regular Piezography K7, as well as Digital Negatives with one printer by just selecting the correct curve for the ink and media you wish to use. The drawback to these models are the quirky mechanics, and there is a uncomfortably high amount of problem reports due to de-laminating print heads, failing sensors and other mechanical issues- so if you get a x900 printer, we STRONGLY recommend you get the extended warranty to protect yourself for the high chance of problems you may experience with these models.
I hope this helps you choose which printer model to purchase, please let me know if you have questions or there's anything else I can help you with.
Best regards~ Dana