My first 'big rig' after a Flyer was a used rig from this forum. I was lucky to get a compete setup:- a 3A arm, a 3A vest and a hybrid sled from Nikolay Kerezov.
The sled has been great for me and I've consistently found work (at my level) and improved my operating every day. The monitor and electrics handle everything from composite to HD. The stage is original 3A, the electrics are Betz, the monitor arm is GPI and the battery hanger from MK-V so it could not be more of a frankenrig. The arm can handle more than I can fly comfortably.
I bought a bunch of cables from Terry West - you can never have enough cables, gaffer tape, electrical ties or velcro!
Most of my work is Alexa or Red with the occasional C300 and I have not had any issues (apart from the occasional minor dovetail palaver). If you have enough of the above (cables, gaffer tape, electrical ties, velcro) you can solve any problem.
No the prettiest rig but I have had the opportunity to sample the work of many suppliers.
I also bought a used Bartech and have a couple of video options (Camwave and Paralynx) on the menu. I haven't had a shoot recently that didn't expect the operator to be able to source these.
I bought a Klassen vest (again from Fabrizio on here) and that is my favourite bit of kit.. ever....
I've built another 'frankenrig' in recent months which allows me to adjust the stage on the fly and has a longer post & more robust gimbal. I need a few more cables (and spares) before using it for paying gigs.
I have enough PAG L95 batteries to power my rig and accessories for two days. I found these easy to pick up from Ebay over time. You can check the amp hours before purchase so you have a pretty good idea of what you are getting. If I were do consider batteries from scratch, I would go with v-mount as these are more common on UK sets. I do have v-mount to PAG plates in case of emergencies though.
I'm currently dithering over my next steps - either a backup arm (I have 2 or 3 of everything else) or an arm upgrade. I'd like to think that an arm upgrade will improve my operating but truthfully, time and practice will do that too! If I upgrade my rig or arm, I can bring my Klassen vest, Camwave and Bartech with me. I think all the old pros here will tell you how important modular kit is.
Box your weight - don't invest 30k and think work will come because you have a big rig. My advice is start with a workshop after playing with smaller rigs. When you are ready to step up, buy some older but battled hardened kit and make your mistakes on the way up. Don't buy a big rig and offer to work for free - you'll get caught out and your karma will be ruined. The worse the rate, the less enjoyable the gig every single time.
Above all, don't ever ask questions on here at night when you've had a few beers/wines. Someone will be needlessly rude to you, you'll tell them to 'foxtrot oscar' and you'll wish you hadn't
All the best,