Hi,
This shouldn't be too much of a problem as Sigma and others such as Metz make dedicated flash units to fit the Minolta ADI system.
Also, since Sony has taken over the Minolta DSLR business, their DSLR flashes should also fit your camera. Their DSLR lenses shuld also work with your camera, plus those from Sigma, Tokina, etc.

Also, please be aware that most of the old lenses for the Minolta Maxxum systems will work with your 5D, so watch for bargains.

One of the reasons I went ahead and bought my Maxxum 7D was because I had so many lenses and equipment for my Minolta Maxxums, that it would have been foolish (not to mention costly) for me to switch systems now.

Hi.
If you have a photo dealer in your area that you have done business with before, you could visit the shop and see what is available in the shop. Maxxum lenses have been around for years, and most are very good, and you may be able to find some good ones at a bargain price. Steer clear of lenses which have unknown--or strange-- brands. Sony lenses are good, but be careful when buying them; they are made for a specific sensor size. You may get vignetting if the sensor size is different from yours.