Strictly speaking, the camera sensor in the D80 and D200 are identical. The images produced by both are essentially in all respects identical. So the friend who told you the D80 had a bad sensor obviously has a different opinion than your other pro friend who uses the D200 every day . In fact the D80, being a newer camera than the D200, might arguably have better image quality due to slightly tweaked/improved noise reduction algorithms.
Anyway, you say that you are finding the D50 "limiting" as you are improving, can you articulate that in more specific terms? And where the (specific) features of the D80/D200 or even the D90/300 for that matter are addressing or improving those limitations? For example, if you're shooting mainly portraits and maternity photos (I'm assuming indoors or in pretty controlled circumstances here), what specific benefit is the enhanced weather sealing and more robust build quality of the higher cameras doing for you? What usefulness will you derive from the ability to bracket more than 3 frames using a range wider than -2/+2 stops? Is 5fps on the D200 going to be better than the 2.5 you get on the D50 in your portrait shooting? Will the D80/D200's ability to act as a Nikon CLS commander be of benefit to you, or would you be just as effectively served by buying a SB900 hotshoe flash to act as a commander module instead? If you can clearly answer these questions then perhaps an intermediate move up to a D200 might be smart, otherwise I still say it's better to save up your money and put it towards the purchase of a D90/300 (or even beyond, say D95/D400?). I don't mean for this to sound derogatory, but I guess IMO if you cannot outline specific benefits in better hardware fixing specific needs in your shooting requirements, you may have to re-evaulate your opinion that the D50 is limiting to your growth.
In addition, this applies doubly so if you are intending this to be a business investment. pdxflint got it right in the 2nd reply where any purchase made for a business should be viewed in the light of "will it make me more money, more than it cost?" If you're shooting for hobby purposes by all means don't exclude the "fun factor", but as I'm not clear whether you are shooting for profit or for fun (or partly both), it is something you should consider.
Regarding the off-camera flash: if you have not already, go to http://www.strobist.com and look at, read through, even practice with their articles and tutorials. It pretty much explains everything to someone clueless regarding off-camera flash, and does so in a pretty straightforward and effective manner. If you're serious about portraiture as a photography business, learning to control lighting properly is going to be 1000x more valuable than any body or lens you can buy.
Ruahrc
Anyway, you say that you are finding the D50 "limiting" as you are improving, can you articulate that in more specific terms? And where the (specific) features of the D80/D200 or even the D90/300 for that matter are addressing or improving those limitations? For example, if you're shooting mainly portraits and maternity photos (I'm assuming indoors or in pretty controlled circumstances here), what specific benefit is the enhanced weather sealing and more robust build quality of the higher cameras doing for you? What usefulness will you derive from the ability to bracket more than 3 frames using a range wider than -2/+2 stops? Is 5fps on the D200 going to be better than the 2.5 you get on the D50 in your portrait shooting? Will the D80/D200's ability to act as a Nikon CLS commander be of benefit to you, or would you be just as effectively served by buying a SB900 hotshoe flash to act as a commander module instead? If you can clearly answer these questions then perhaps an intermediate move up to a D200 might be smart, otherwise I still say it's better to save up your money and put it towards the purchase of a D90/300 (or even beyond, say D95/D400?). I don't mean for this to sound derogatory, but I guess IMO if you cannot outline specific benefits in better hardware fixing specific needs in your shooting requirements, you may have to re-evaulate your opinion that the D50 is limiting to your growth.
In addition, this applies doubly so if you are intending this to be a business investment. pdxflint got it right in the 2nd reply where any purchase made for a business should be viewed in the light of "will it make me more money, more than it cost?" If you're shooting for hobby purposes by all means don't exclude the "fun factor", but as I'm not clear whether you are shooting for profit or for fun (or partly both), it is something you should consider.
Regarding the off-camera flash: if you have not already, go to http://www.strobist.com and look at, read through, even practice with their articles and tutorials. It pretty much explains everything to someone clueless regarding off-camera flash, and does so in a pretty straightforward and effective manner. If you're serious about portraiture as a photography business, learning to control lighting properly is going to be 1000x more valuable than any body or lens you can buy.
Ruahrc