I have played guitar for about a year now and I want to play bass now along with guitar. There are many reasons for me wanting to play bass. I will already have a bass amp in a few weeks from my friend who says the amp is a decent amp although i havent seen it yet. Also, I play rhythm guitar and i am very rhythmically minded and bass is, of course, a rhythm instrument. Plus, I think joining a band would be much easier because I know about 5 drummers, 10 guitarists and only 1 bassist. Anyway, my budget is about $300 and I would definitely take used. I would prefer something that is of decent enough quality that I could play with it in a band and that has a decent metal tone in the style of deftones, system of a down and tool.
1. Strap on some overalls
2. Arpeggio Seriousness
3. Lock Horns
4. Take a big fresh breath of air
5. Go Surfin'
6. Get high on fire
A used Ibanez SR would suit you fine.

But rhythm guitar =/= bass. I made that mistake, and I encourage you to not. Bass is more aligned to the drums and requires a greater dedication to the groove and the beat than rhythm guitar ever does.
Ibanez is the greetest! I love the sleek designs, very easy to play. Another decent pick would be a Fender P-Bass of some sort, it might be cheaper. And it's true what Anarkee says, the bassist is more aligned with the drummer. But bass players are more scarce than guitarists, so it's a wise choice.

Getting the right bass tone has more to do with technique than guitar, be sure to study the playing styles of those bassists. I think they used finger-style, popping and slapping to get that clacky sound. Experiement with strings as well, maybe try flat-wounds.

Down the road you should consider investing in a bass overdrive pedal and a compression unit if the amp doesn't have those features.
Same answers everyone gets when asking in this forum.

Ibanez SR
Yamaha RBX 375
Squier VM
For how can I give the King his place of worth above all else
when I spend my time striving to place the crown upon myself?
Might be worth finding out what the amp is as well. If its a 20 watt practice amp you aren't going to be heard in a band situation. A good number of metal bass players use some kind of OD or distortion to get a more agressive tone too
Peavey is also pretty good, and very reasonably priced.
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Bassists don't hover on the forum day and night like guitarists. We've got lives to lead, music to play and whiskey to drink.

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I'd rather go at my hands with a hacksaw than play lead guitar, and I'm only slightly exaggerating.
Since you want it for metal I'd recommend one of the Dean Metalman guitars. They look and sound killer.
If I could get THAT sound out of a Casio Mickey Mouse watch, I would play a Casio Mickey Mouse watch