The nic base will either be PG or VG base.
As a fairly low nic user, you might just want to settle in with 36 mg/ml for your nic base. GoodProphets has pretty frequent sales on their 36mg/ml, either VG or PG base. Otherwise check MyFreedomSmokes.
For non-PG-based flavorings, natures flavors Organic coffee flavoring system flavors are pretty good. In tobacco, I use perfumersapprentice tobacco blend alc (a mix of their tobacco absolute and ethyl maltol in pure grain alcohol, I believe). From them you can also get liquid menthol concentrate (or you can use plain menthol crystals, but I find the concentrate simpler, good consistency), and they have several other tobacco-oriented flavorings. FaeriesFinest has some decent non-pg mints.
I'm a high-flavor mixer, and I'll still top out at about 20% on flavorings, rarely go higher. Since you vape 50/50, you should be fine using the PG-based flavorings, too, unless you also have a strong desire to use organic flavorings (I use the NF organics because I DIY for some people with PG sensitivity).
As a 50/50 person, in the nicotine I'd probably go to the 36 mg/ml PG based -- it's thinner, simpler to work with, in general. You would be using it at 16% (5 ml per 30 of final eliquid) to achieve your desired 6 mg/ml final strength.
To keep it simple, go to GoodProphets first. Pick up some of their 36 mg/ml PG nicotine base, plus some pipettes and some of their graduated cylinders/beakers, plus empty bottles. Check out which perfumers apprentice (aka, "TPA") flavorings they have, see if they're carrying the liquid menthol concentrate and pick up some of the tobacco flavorings and any others that sound interesting. Perhaps buy a bottle of plain PG from them, too.
For VG, get it at a Rite Aid or CVS drugstore. You want Glycerin USP Skin Protectant, a squat, clear, 6 ounce bottle in the bandaids area, costs about $5. The ingredients list will only say Glycerin 99.5%.
Check out the stickies in this area, download scubabatdan's Excel calculator or one of the others mentioned and have fun
Cost? The pipettes, graduated beakers, syringes (if you chose to use them instead of pipettes) last a LONG time, with cleaning, and are pretty cheap anyhow. As a brand new person, particularly since you haven't settled in on which flavors work best for you, maybe won't get nicotine base on sale, you'll likely be paying more than I do, but even early on my DIY cost was about $4.25 per 30ml for 24mg/ml mixes at high flavor. My cost nowadays, knowing which things I like to work with, restocking on sales, etc., is under $3/30ml, and for 6 mg/ml noticeably less. So, my eliquids cost a lot less than even lower cost commercial, and I have really good consistency, control the product. DIY isn't for everyone, but if it works for you you'll get really good eliquid at really low cost.
Overall, for tobacco base flavor, my own favorite is Seedman Commercial cigarette concentrate. I use it for "real" tobacco mixes, though the TPA tobacco blend is nice for flavored things (like vanilla tobacco). Tastes vary.
(multiple good responses before mine, but I'll leave this up since I wrote it
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