Nikon d3100 speedlight vs strobes?

I have a nikon d3100 but I also use 35mm film cameras and have a sb400. I have a $300 limit.
but I want I flash or some sort of lighting system I can use for both of my cameras, I want to be able to shoot weddings, portraits, and live bands with this. I can sell my sb400 to up my limit. I was thinking about getting a beauty dish too just because I think they're cool but I can't get one for a sb400.another problem with the sb400 is it casts a shadow of certain lenses. And advice will help!

Added (1). Also I have a way of getting a sb600 for $175, which I kinda want rather than the sb400 I have now.

For serious flash photography, you should invest SB 700 and above which has wireless flash trigger.

Set your SB400 to manual mode, and see if it works with your mechanical film SLR. Even if you are planning to replace the SB400, that test will probably tell you how an SB600 or SB700 would behave with the film camera.

Speedlights and electronic flash units are designed for repeatable TTL artificial lighting attached to a camera or in a studio setting and are colour balanced to a specific standard.

Strobes are continuously cycling flash units designed to fire at specific rates. They are used mostly in scientific laboratories, concerts, theatrical stages or night clubs. Color balance will vary with the quality of the strobes used

If you attempted to use a Speedlights and electronic flash in this way, it would soon overheat and damage the flash tube

Now that the new SB700 has been released, I have seen used SB600 units selling for under $250 on craigslist.

Invest in a SC-28 off camera cord for your SB-400 (used SC-17 will also work just not lock into your flash shoe), add a bracket and you are good to go. The cord + flash + bracket combo will get the flash up and off the camera to eliminate the shadow cast by the lens.

You are not going to buy a new flash with a beauty dish within your budget but I bought an old Bowens with beauty dish for about your $300 (so start looking, that's like part B of my plan)--you can use your SB400 to trip the studio flash--to be safe do not hook any non-Nikon flash to an electronic camera, SLR or DSLR. If your 35mm kit is Nikon your SB400 should work with it. (And if that's a NEW SB600 for $175 grab it, I gave $200 for mine.)

You should buy this one

Nikon SB-400 AF Speedlight Flash for Nikon Digital SLR Cameras

Compact, lightweight Speedlight unit
18mm angle of coverage with Nikon DX format digital SLR cameras
Bounce capabilities in 4 steps up to 90 degrees; flash shooting distance range from 2 and up to 66 feet
Approximate guide number of 98.4 feet at ISO 200 (at 18mm zoom head setting)
Operates on two AA-size (1.5V) batteries; fast 2.5 second recycling time