There are many options to get there, but first you really need to know where you want to go...
Are you looking at studio work (portraits, still life, etc.), or more location / landscape?
Those will impact both your camera (monorail versus field) and lens options (e.g. wide=> may need bag bellows capability, long=> may need a lot of bellows or telephoto lens design, large heavy portrait lens=>need robust front standard).
Do you have a LF (4x5) enlarger that you are using for your MF darkroom work, or will you need to buy one, or looking at contact printing?
Short answer until you answer the above questions yourself, is to befriend a LF photographer who is photographing the types of subjects you want to photograph and see what works for them and what does not work, and what they would suggest.
If you are using a MF system, you no doubt know the cost in acquiring all the extra bits, besides the camera and one lens, so you are better buying someone's kit, than trying to assemble one yourself from scratch.
The good news is that as a result of the move of many professional photographers and serious hobbyist to digital, good quality equipment is much cheaper than it has been, and people are getting out, as well as others like yourself getting into LF...
And the other good news is that if you are patient and careful in your purchase of used equipment, should you decide to abandon LF, you can re-sell the equipment and just consider the difference in purchase versus selling price, rent on the equipment to experience it.
Do be aware that for some of us who have tried LF, it can cause an addication type of behaviour, and you will find yourself pondering bigger and bigger formats, until you find yourself not just looking at the APUG LF section, but also the ULF section too.
Good luck with your search,