Basically, there are three following kinds of 12.1" 16:10 screens:
1) TN-CCFL 1280*800 garbage lenovo used to put in their X200/X201; not-that-bad TN-CCFL 1280*800 from other manufacturers (e.g. samsung); IPS-CCFL 1280*800 HV121WX4 from boe-hydis (comes in three models, -100 and -110 being glossy and -120 being matte); *VA-CCFL 1280*800 LTN121AP02 from samsung.
2) TN-LED 1280*800 garbage lenovo used to put in their X200/X201 (different connector).
3) TN-LED 1440*900 lenovo used to put in their X200/X201 (physically different panel).
4) IPS-LED 1280*800 HV121WX5 from boe-hydis, *VA-LED 1280*800 LTN121AP03 from samsung (different connector).
Switching between different panels of the same kind is simple as plug-and-play. Switching between panels of different kinds is significantly more complex, sometimes involving replacing a lot of components, sometimes requiring to replace the whole upper lid assembly, sometimes requiring you to have good skills with soldering iron, and sometimes impossible at all. The other users probably know better about switching between kinds.
So, basically, with plug-and-play there is no way to satisfy your last condition, as all panels except for the single panel of the third kind are 1280*800. Also, none of the panels of the first kind come with LED-backlight (which is quite understandable, as LED requires different power supply).
Also, i believe, both TN sorts of the first kind are quite bad (with stock lenovo being "try-to-find-anything-worse-than-that" and others being slightly better than that). Basically, you have to choose between boe-hydis glossy, boe-hydis matte and samsung one. All three are just about as bright as the stock TN panel. Colors are nearly unacceptable IMHO on LTN121AP02 (though it might be just my bad luck), and on boe-hydis these are quite reasonable.
So it seems that if glossy suits your needs, HV121WX4-100 and HV121WX4-110 are the only options you have (and, as HarryC said, beware of the "compatible" panels). I second HarryC opinion on thinkpad_2007 ebay seller, you can safely purchase the screen they offer.
If the matte is a must, and you want to keep your X200, then you have to choose between LTN121AP02 with terrible color reproduction and HV121WX4-120 for $400. However, there are other routes:
1) You may as well sell your X200 for about half a price of new X220 and purchase an X220 with IPS instead. The difference in price probably won't be that much higher than $400 HV121WX4-120 is offered for, and you will get an exceptional brightness and contrast you would never get on HV121WX4, along with much better performance, on-board DisplayPort, increased battery life and many other features.
2) Also, you may consider switching from ThinkPad to something else. If viewing angles are not a concern for you, some TN may also fit your needs. For example, W520 FHD screen is much better than HV121WX4 or LTN121AP02 unless it comes to viewing angles; the resolution, color reproduction, brightness and contrast are perfect compared to both HV121WX4 and LTN121AP02 (unfortunately, i haven't had a chance to compare W520 FHD with X220 IPS). Even some old "netbooks" have the screens
much better than LTN121AP02. Probably, there are some laptops with such great TN screens and higher resolutions in your size-weight range.
All than fun with replacing X200 screens was popular until X220 came out. Now, when X220 are so cheap and their IPS screens are so good, it is hardly worth it to replace screen on an old X200.
They do reflect a lot of light in bright areas.