How important is the tonearm?

VERY!!

out of my league

Matching a cartridge to the tonearm then choosing the right table to hopefully hear the arm,cartridge and your music at their very best. It's all important this far up the stream.

Remember when Ivor Tiefenbrun popped onto the scene back in the 1970s preaching about the source ,.......whom here bought into that back in the day? :-o

As Raul says....the Jelco is inexpensive.
It doesn't mean it's a bad arm. If you find a cartridge that you like and which is a good match, you don't have to pay more for an arm.

I worked a number of years ago in a speciality business where all we sold were phono cartridges and tone arms. There are some basic principles

1)matching a tone arm and cartridge is like matching a car and a shock aborber... what you want is for the arm to "hug the road". That translates into lightweight soft suspension cartridges should be in light tone arms, heavy stiff suspension cartridges should be in heavier arms. A mismatch means wow and flutter, poor tracking, a suspension that bottoms out on imperfectly flat vinyl.

2) the bearing assembly is critical, arm should move easily up and down, no wobble. the meet point for the bearings should be machined to high tolerance with the highest durability steel

3) the easiest test of a tonearm/cartridge match is a female vocalist on direct to disc. My favorite used to be Amanda McBroom on West of Oz (Sheffeld Labs) With a bad match the voice will become edgy or break up, with a good match the voice will remain smooth in the most intense passages. This tests for two things, trackability and tone arm resonance. Note that the more complex stylus shapes require correct VTA adjustment for this test to be valid but elliptical and hyperelliptical shapes are fairly forgiving.

As to importance, the tonearm is secondary to the turntable. A spring suspension turntable like Linn or Sota with an entry level tonearm will reveal much more than if you buy a better arm but compromise on a non spring suspension turntable.