That's a hefty budget for a complete small refractor setup, even going new for some items. A good complete refractor setup will be quite doable. I guess the first question should be about the mount.
Do you want/need something very portable or do you think you you will be looking at a bigger scope later? For a refractor up to around 80mm, something like the Sky-Watcher Star Adventurer GTi is quite suitable, but something like a HEQ5 Pro will give you a more stable base with greater capacity.
As for the telescope, you have a very wide choice from many different manufacturers. You will need the scope, a flattener/reducer plus a T-ring to suit the D90. That and a mount is the very basics to get going, but you will probably want a guide scope & guide camera for better tracking.
If I was starting from scratch, and this is only my opinion & you know what they are worth, I would go something like:
Scope: SVBony 80mm ED (SV503) + flattener/reducer (SV193) + M48 T-ring for Nikon. That's buying new (~$1000) - just look for similar if going 2nd hand, you will find some good deals in the classifieds here.
Mount: 2nd hand HEQ5 Pro or similar, preferably with belt mod. Again, watch the classifieds.
Guiding: ASI120MM Mini guide cam + SVBony or ZWO mini guide scope.
Filter: you may wish to invest in a filter to help deal with any light pollution - not needed but can help (a lot). These can be had for very little or a small fortune, depending on what you want.
Then there are the 'extras' - some method to power the system (I run totally off 12v), a dew heater to fit the scope, a bahtinov mask to aid with focussing, a method to connect to pc/laptop (cable or wireless - connection and control software is readily available for free) and probably something else I've missed.
That should all come in under your upper limit & give you a good, usable setup from the start that, once you get comfortable with it, will provide you with great results.
Hope this helps - I'm sure others will come along with their own suggestions, pretty good bunch here. I'm sure someone will come in with a newt or MAK suggestion...
Cheers,
Mark
