Nikon EH-5d AC Adapter

The D700 uses the old EN-EL3e battery, so coupler is not needed to attach this EH-5D to the D700.

The Nikon EH-5d AC Adapter paired with the correct dummy battery for your camera will not lose power.

The Nikon EH-5d AC Adapter is multi-voltage. All you need is a plug adapter not a transformer. This is one of many plug adapter options, https://bhpho.to/2TomikJ . Note you need power supply connector to use that for your specific camera. There are others buy this is some, https://bhpho.to/2TpJ9wr

Yes, it is compatible with the EP-5A.

The Nikon EH-5d AC Adapter (B&H# NIEH5D) can produce the same amount of power even when used for long periods. However, higher temperatures could cause an increase in impedance and therefore a slight reduction in voltage output from the MOSFETs. As such, we'd recommend making sure to allow the transformer's heat to escape during use rather than, say, covering it with anything that might insulate it, leaving it in the sun, or otherwise preventing it from touching surfaces that have the potential to wick heat away from it such as a cool floor. Since something similar can happen to your camera (which has the potential to cause or contribute to this problem, too, even if no overheating is sensed nor warned about), you may also wish to leave the battery door of your camera open, the rear screen pulled or at least tilted away from the body of the camera, and your camera mounted to something that can serve as a heat sink such as an L-bracket, cage, or tripod. Shading the camera from direct sunlight is also helpful. This product may be mounted in the hot shoe and used for that purpose even in slightly windy conditions: Glide Gear Adjustable Hood Camera Lens Shade with 11" Friction Arm (B&H# GLLS100) https://bhpho.to/3t1BhCV

I haven't located a power adapter either for the Nikon D90 either but would double battery life help you? The Nikon MB-D80 Multi-Power Battery Pack officially discounted is usually on sale here on B&H used for $49. It won't allow you to continuously use the D90 plugged in but it holds TWO regular Nikon batteries at once. (or 6 double AA) - I own two D5s, a D850 etc and IMO the D90 still punches well above its weight. It is perfectly sized extremely solid but lightweight. Its bigger FX brothers may pack a lot of features but value for ounce the D90 is extremely hard to beat. Best of luck.

my understanding it also needs the EP-5B power connector

The ac adapter is 9v.