Also, be aware that the Kenwood TS-520/530/820/830 hybrids are CW and SSB only. I don't know how much AM (or FM) matters to you. They also don't do general coverage receive.
I own a TS-430S too, and I wouldn't sell it. Two things to be aware of though: FM mode is an option (requires the FM-430 board), as is the 6Khz AM receive filter. Both of these can be found used without too much trouble, but I'd try to buy a radio that at least has the FM board already installed.
The TS-140S and TS-680S (same as 140 but with 6 meters) are nice radios too. They're dead simple to operate and decent performers. The only issue I've seen is that some people find the slider controls on the right side a little difficult to adjust, particularly the power control. Note that the TS-680 only does 10 watts on 6 meters. I see them sell from $350 to $400.
Note that there are computer control options available for the TS-440, TS-140 and TS-680, though not for the TS-430.
The TS-850 is the best out of the ones you mentioned, however it's a little out of your proposed price range. Cheapest I've seen one go for was maybe $650, sometimes up to $800. I think the TS-450 goes for around $500. I think between the two, the TS-850 has a better receiver (it also has the VBT tuning option normally found on the bigger rigs). If you ever consider a TS-850, ask for the serial number before you buy. Radios with 40million or better serial numbers are your best bet.
By the way, don't rule out the TS-440's big brother, the TS-940. The TS-440SAT may go for around $450, but sometimes you can find a TS-940 for just a few hundred more:
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That's not a bad deal especially when you consider that you don't need to buy a separate power supply. (Though that also makes the radio heavy.) Some people have complained about bad solder joints on TS-940s though, so a little research before buying is recommended.
Unfortunately, some of the best Kenwood rigs (IMHO) are also the most expensive. The TS-950SDX is the king of the hill, but they typically sell for between $1500 and $2000, and that's if you can find someone willing to part with one. The TS-950S is much cheaper (sometimes around $800), but lacks the DSP features that make the TS-950SDX so desirable.
There's also the TS-870, which is a popular choice with ESSBers. If you do a simple mod you can feed it audio from the rear panel ACC2 port and achieve ~4Khz frequency response, but it's generally cheaper than the SDX (~$1100). For that money though, you could also buy a used TS-2000 and have HF/VHF/UHF all in one box. (The TS-870 is HF only.)
-Bill