Just my 2 cents as someone coming from a similar position...though many of the people on this forum are extremely experienced and have much more knowledge of location gear.
I'm a post guy in NYC, but take on some occasional location gigs, in which i usually rent a kit from a few different friends. I've been trying to maintain about 1 location gig/week to supplement my post work simply because it's nice to get out from behind the computer and also meet new people (which I've made a lot of new friends and colleagues through), and it's a nice lil wad of extra cash. Recently, it's gotten to the point where I've begun building up more of a network and it's kinda of a pain to constantly rent kits and make 2 round trip treks in NYC to my friend's places, so I've been slowly building a kit.
My current, though incomplete, location rig is a 744T, Mixpre (original 2 channel), Sanken cs3e, a few NP-L7S and charger, BDS v2 and NP-1 Cup, Petrol bag, pistol grip, softie, sony headphones. I've bought everything used so far, and have had good practical experience with all of the equipment before purchasing. Still need to get a pole and some wireless/lavs but pretty close to a self-contained kit for corporate/industrials/commercial work and Doc stuff (and pole/lavs are much easier for me to quickly grab from a buddy or rent from a shop if needed than a whole kit).
I was also considering the Mix-Pre 6 but I felt that I would outgrow it pretty quick (can't generate TC, no internal drive or dual card for backup/failsafe, not as great pres, etc) so I waited until I found a good deal on a 744T (which I had used a lot) - Side note, my buddy in Brooklyn is selling a 744T on the production gear buying/selling group on facebook. I'd look for something like an MKH50 or a Sanken CS3E first (MKH great for interior reflective rooms and pretty good reach, CS3E also very good close up interiors and better reach for wider shots, and noisy rooms, and excellent outdoors...lot's of punch, but maybe not as sweet/thick as the 50, you need to be a bit more on top of their voice as it's more directional). Get a cabled graphite pole, or get a super super cheap used aluminum that will get you through enough gigs until you can afford the graphite. Trew Audio has some used NP batteries/charger packages for sale as well as a bunch of cheap used bags (also Gotham for bags). I'd try and save for 200 series lectros (maybe they can throw in cheaper lectro lavs or trams) and then save up for some COS 11 lavs. But otherwise the Senn G3 wireless sound decent to me and have worked in tough radio environments including a live broadcast sporting event at Barclay's Center, and you can find those used all day (and sell them for pretty much the same). All of these items are pro grade (aside form the G3's maybe), will last a lifetime, and you can upgrade with, as opposed to having to sell or outgrowing.
More importantly, I think the main advantage as far as garnering location business is actually your post experience. Editing dialogue and production sound, mixing ADR, and being a sound effects editor gives you a very good perspective on what will work in a scene, how it might play on a mix stage, and potential issues that you can actually rectify by simply making a suggestion to the director or DP (in a very friendly and in-often manner), which makes your value increase as a boom op/mixer increase. Additionally, I've found that the detail and concentration required for editing dialogue (hearing subtle sounds of saliva cackling in someone's mouth, a boom going off axis, lav rustle, etc) actually makes you very attentive to the voice on set while booming/mixing/monitoring, and that generally comes off as you being very professional and dedicated. Plus, from a post-perspective, you know that as people's voices rise and fall we will end up riding those dia faders or having to compress, so simply pushing in and backing off the boom as they speak helps maintain a more consistent volume and makes thing sound better and easier to mix in post.
Back to gear...I buy a lot of guitar/music gear, almost always used on craigslist and such. Buy things that will last a lifetime and that you can grow with, but don't put too much thought on the gear cos it's super easy to go down the blackhole of spending days perusing gear retailers/ebay/craigslist/reverb/forums. The fact is, a talented individual can make low-budget gear sound good and an untalented individual can make great gear sound bad.