Hi. There are more factors to it than just 1200 dpi. By the way Phaser 7800 / 7500 can print better quality than other 2 you mentioned. Every printer in your list has its own pros and cons..
Phaser 7800 / 7500 has really good print quality, nice bright prints but it puts lines on high density areas from exit rollers especially with glossy media and there is no way to overcome that.
OKI ES3640 is nice durable machine, we used one in our shop and sold (We're OKI Authorized Service Partner) hundreds of them in past years
Similar to Phaser - these printers are designed to be self maintainable for the most part (You can change Fuser/Drums really easy but I would not say that OKI is Up to the task as far as Photobooks especially on glossy it tends to lay a light background, some stocks show that less than others but at my shop we always had OKI because it can happily output flat cardstock with better registraton and Phaser 7760 to print on Text weights - posters etc. because of it's quality.
I do not have hands-on experience with Canon C1 but I heard that it lays awesome color if kept in good condition and there are external fiery available to it which will give you much better control over color and pretty much everything else. There is a version of it Called C1+ with clear toner, should you decide to go for it, go for C1+ as it will give you a chance to offer some effects like semi-raised print or something similar to Spot UV. One thing important - this is not a self service unit - everything you might need is a service call, make sure you have a knowledgeable provider in your area, better yet - get a service contract.
We have few OKI ES3640 for sale should you choose this one anyway $1800, we also have OKI CX3641 which newer model and does prints a little better quality than ES3640, $2200.
I have an offer from a leasing company we deal with on Canon ImagePress C1+. It has about 238K on the meter and equipped with RADF DUP LCT 4 DRW PRINSERVER H1 FIN: AA2 $4600
If you are in PA we can deliver and assemble (no staircases with Canon C1) at reasonable cost.
Let me know.
UnlimitedBT at Yahoo.com