Hey people,

I've been playing guitar for some years now and dabbling in bass, now I'd like to jump deeper into the world of bass guitar.

My favorite bass players are Duff McKagan (GN'R) and Krist Novoselic (Nirvana) and I'm looking for a sound that sort of blends the two together. What kind of gear do you guys recommend for something like that?

I have an Epi Blackbird (which is basically a Nikki Sixx sig thunderbird) and a Hartke A70. I have a significant guitar rig (OCD, fuzz, chorus, etc...) but it gets more complicated because I'm starting a band with my brother and I'll most likely let him use my rig for that.

I don't wanna spend more than I need to. (Although, hypothetically, if I were to buy a second bass guitar on the low end of the price spectrum, what would be a good option? I'm thinking Sq. P bass or something like that).

Call me Kerouac
I prefer Fender basses, specifically Jazz. The nice thing about a Jazz bass is that you can make it sound like a Precision simply by turning down the volume on the bridge pickup - but with the two pickups you have more versatility than a P-bass.
You cant go wrong with a Jazz or P bass for genre's such as Guns n Roses and Nirvana, there's plenty of video's on youtube to check out the sound off Jazz and P basses.

Also check out the Washburn T24 and the Ibanez SR300/SR500

Here's a channel of someone who cover pretty much everything with the Washburn T24
I'd recommend a Jazz over Precision, or at least a Precision with P/J configuration- Duff played Jazz Bass Specials back in the day and his sig is basically the same but P/J instead of 2 Jazz pickups. I reckon if you snoop around on ebay or Reverb you could find a decent secondhand one, but Squiers aren't too bad quality these days. Get one and stick some Seymour Duncans in it?
Squiers are a safe bet nowadays, P or J. I wouldn't worry so much about the sound differences too much the necks are different with P necks being chunkier than J necks. I play my J at home because I prefer the sound but with my band I end up with the P most times as there's something about how it sits in the mix. You get some P's with a J neck too so you need to play them before you buy.

The Hartke is going to struggle with a drummer and a whole load of other instruments. You need to look for something in the 200W region if you want to gig. I'd always advise going used for a first purchase so you can move it on at little loss to yourself as inevitably we all want to upgrade as we learn more. If you can handle the weight there's a lot of old Peavey gear out there that is bomb proof and sounds OK for not much money.