I have built a few homemade sawmills, had a few bandsaw sawmills, and have an old rotary sawmill, so I know a few things about sawmills.
The best sawmills you can buy are Woodmizer, but they are expensive, however, like most things in life, you get what you pay for. Finding a used one at a good price might be possible though.
You want to stay away from chainsaw mills as they are just too slow. You will start a log today, and finish it tomorrow...they are that slow. And they take too much of a kerf. Goodness, a bandsaw mill means you lose a board to sawdust after cutting 15 boards, but with a chainsaw you lose one board to sawdust after just three boards. It is just too much waste for the slow speed that sawing lumber is.
You want to stay away from the Norwood brand of sawmills. They are a lesson in frustration. The frames are too weak and so when you set a big log down on the mill, it flexes and thus cuts banana boards because as the boards are removed from the log, the weight relaxes and the flex springs back. The head-rig is also located too close to the left track, and so you have to have really straight logs with no stubs or buttresses too the log. It also lacks the ability to cut on a taper. It really is a frustrating sawmill to work with all the way around, and I was glad when I watched mine leave behind another persons truck, but still felt sorry for the guy.
The real important part of a sawmill is how the log is secured. As some have said, hydraulics are the answer, but it is an expensive option. But some are just too frustrating, like the Norwood which is slow, inconvenient and has few adjustments.
I have heard great things about the Harbor Freight Sawmill, with the exception being the track. As cited in the paragraph above, this is important so I think the best bang for the buck spent, is to take the Harbor Freight Sawmill ($2000) and make it what it aint. My suggestion is to buy the plans for the Procut Sawmill, which is a company that sells plans to make a homemade chainsaw sawmill, but just build the track the plans show, as the chainsaw aspect is worthless. That would give you all the quality of the saw rig of the Harbor Freight sawmill, but put it on a 24 foot track with log holders and adjusters that rival even the Norwood Sawmill. If you are unable to fabricate, since you have plans to work from, you could just have a local metal shop build the track for you, and still be ahead. This would give you a sawmill that was quite capable, and be in the $3000 range.