My .02. You didn't say whether the keyboards are primarily live use or in a home studio. That can make a big difference of how to move forward. If it is for live use, weight is seriously important. Find a light 88 key instrument, and put up with the not-so-perfect keyboard action. Then put modules around it. Your back with thank you. For live, I use a Yamaha MX-61, and take the P-12 module with me. I have an 88 key Casio, but I gravitate to the MX-61 primarily for weight reasons. The Casio is about 30 pounds, and the MX-61 is 6. It's a big difference for me and my old back.
Assuming home studio, I would start with a keyboard which you would use as your main controller. Weight is not really important in that environment. The Kronos would be a good choice. Start building around that keyboard. Once you have that instrument, it is not going to matter so much whether you get modules or keyboard versions surrounding it. You will probably use the main keyboard as the controller regardless.
The Kronos is really first rate. However, you may want to investigate the Montage too. The Montage is more synthesizer oriented than the Kronos. Pluses and minuses for each instrument. Like you, the key bed feel is very important to me. The Montage 88 key bed is the most piano like of any electronic instrument I have played. So, it is worth looking at. I bought mine "used" at Guitar Center - a store demo. It was in perfect condition. Based upon the serial number, it was about three months old when I bought it. With the Guitar Center 1 year warranty it cost me $2850 plus tax total.
Whether you buy a Kronos or a Montage, both are very deep instruments. You can learn a lot about analog synthesis from the Montage. Not so sure about the Kronos. Also, the Montage has a full 8-operator FM synth engine similar to the DX-7, but with more modulation capabilities and 8 operators (6 in the DX). So, it can also fulfill the FM synth duties too- and much more user friendly than the DX. There will be a lot to work with and a lot of time invested learning either synth. In the meantime, you can save the money for the next synth.
Another alternative is the Yamaha S-90. It's an old synth but a really good workhorse. The key bed is on par with the Montage. However, some of the early S-90's had problems with their key beds. Yamaha has a non-publicized recall on those. So, be careful if you go that route. The sounds are from the Motif and are somewhat dated. There are some analog synth-like programming capabilities, but it is not a full fledged synth like the Montage. However, you can get one for $1000 or less in really good condition. The S-90 was my controller keyboard until I bought the Montage.
I agree with Paul Dither that a module may be the way to go for the second instrument. I have seen P-08 modules advertised for as little as $700 and Rev 2 as little as $1200. I went with a module for my P-12. No regrets. Even if you buy a synth keyboard, if you are like me, you will gravitate to using the 88 key controller rather than the keyboard on the synth.
I love DSI products. You can see in my signature block, I have three. Very capable synths. However, I would not recommend, except maybe for the P-X (haven't worked with one yet), a DSI synth as the main controller for your setup.