I would recomend getting a cheap camera with full movements, like a Toyo 45F, a few film holders, and one or two lenses in the 150mm to 300mm range. This would be a relatively cheap, and workable, intro to the format. If you like the 4x5 process, there are any number of Germanic cameras available that are pretty incredible.
The difference in cameras, for your purposes, is reallu marginal. If you shot architechture or products the differencs can be huge. But in reality, these cameras are really just two boards connected by a bellows with a lens on one end, and a piece of glass on the other. The differences in systems comes down to accessories, ease of focusing, and portability.
For portraits I like a 210mm (like a 100 or so on a 35mm system). I also like a 300mm. But remember, there are few limits on close focusing. You are limited by the bellows extnsion, so a single lens would be OK to start with. I like a 150mm for environemntal portraits.
For portraits and fashion you will use few movements, maybe front tilt, so I wouldn't be too concerned with movements.
I you like the process and working in the format, I recomend a used Sinar for studio and for portability, a used Linhof Tech IV, V or Master Technika.
I have a Sinar P/F system and two Technikas, a IV and a V. I shot fashion, beauty and portraits with these systems. Rarely did I use movements.
The Sinar system is the best there is, in terms of precision, the number of readily available accessories, and the ability to combine sytem standards and rails to make a camera that is suitable for specific shoot. I prequently used the P rear and the F front standards for lighter weight. I used a a P 8x10 standard. I have 40" of extension rail. I use a bellows as a lens hood. A bellows or two for extension, and a bag bellows for focusing. Want to compose using a corrected image? Slap on the reflex box.
That being said, in teh field the technika's are fantastic. Light, compact. Everything I need for a shoot fits in a large Domke bag: film holders, a body, meter, dark cloth. Changing bag, and aditionl len, a polaroid back. A tripod attachs to the bottom.
Whatever you decide, its a journey. Fortunatly the basic stuff is chap nough to give it a shot with out a major commitment.
Good luck.