I saw this thread a little late, but I'm still going to comment...
I've been riding bikes for almost 50 years. I even raced bikes ALL the way back in the 1970's...BMX. Redline! I also spend time bringing vintage bikes back to life as a hobby. Here is a picture of my current project...it's been going real slow, but I'm starting to make progress. At the end of the day...the frame is like brand new with no rust, so she's a beauty. It's a 1947 Hawthorne. All original.
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In some fashion...my opinion would mirror Jetster's....relatively sound advice he's given from my perspective. I can no longer ride mountain bikes(with my neck I have to sit straight up), but I'm a fan of Both Giant and Trek in your price range. Although, maybe it's where I live, but a solid entry level from either is cheaper than $600. $400 to $500 would be more accurate in my area. I also believe a good relationship with a local bike shop is a must. I can do all of my own work...but I always take care of my local bike guys.
Now...with electrical bikes and your budget. The only bike I could come up with that I would personally purchase for myself in this range is probably the Lectric XP 2.0, not the original, but the just released 2.0. They really listened to their riders and have fixed quite a bit of the shortcomings associated with their first production run. Because of the price, it has a very large(and getting larger by the day) cult following.
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I've ridden both the Lectric XP and the Sondor. While I(and this is subjective) found the Sondor to be a little more aesthetically pleasing to the eye...I did not like the width of the frame. It felt too wide, especially around the crank area. I actually kept looking down because something felt off. I do remember this was the cheaper of the two Sondor models, but it still appeared well equipped. The Lectric XP was the original model, but the bike I rode was highly modified, so it's hard to compare. After 5 miles or so, I wanted one. Still do...
XP 2.0 Black
I have owned many many bicycles in my life. Very expensive bikes and very cheap bikes...but now the only bikes I can ride are beach cruiser types because as mentioned I have to sit straight up. So, I actually prefer those cheap Wal-Mart bikes that nobody likes. My current one is going on 7 or 8 years and still rides like new. Bought it for $45 used from a guy who rode it a half dozen times and parked it....like most Americans. The only saving grace for this heavy bike is...the frame is aluminum, but it still takes the scale into the 40's...high 40's.
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Even with that I say unabashedly...It has to be the best money I've ever spent. I absolutely love this old bike...may not look like it but I've ridden the hell out of it. I also pamper it a bit too...
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I even ride it in the winter...inside of course.
Best,
Liquid Cool