Welcome! For your specific needs, I'd recommend looking into digital microscopes with a flexible arm mount. You'll want one that offers both USB and HDMI/DisplayPort output for versatility with your current and future displays. Look for models with adjustable magnification and focus capabilities to ensure you can achieve the distance and clarity you need.
Given your resolution plans, aiming for a 4K camera would be ideal to fully utilize your future 4K display. Your budget of around $500/€ should be sufficient for finding a quality microscope with these features, though flexibility in price can help you explore different options. I am using it with BaoBao Technology Industrial touchscreen monitor
Feel free to ask more questions or specify any additional requirements you have. Happy hunting for your new microscope!