You're asking this question in a Car A/V enthusiast sub. So keep in mind, like all enthusiast or hobby subs, there is going to be a certain amount of snobbery and brand favoritism. Budget-friendly definitions will also vary, as Car A/V enthusiasts will prioritize Car A/V. I'll try give you my thoughts, as I am inferring from your post that we have similar definitions of budget-friendly.

First, you'll get a lot of mixed comments on Cerwin-Vega - typically from people that don't have them or have never had them. Naturally, people will recommend brands that they've had good luck with. I have 4 Cerwin Vega HED series 6x9s in my car running off stock-deck power, I can't upgrade my head unit without some serious modifications, and likely will never upgrade it.

I knew I would be running my upgraded speakers on deck power and visited many local shops and listened to quite a few brands - on deck power the Cerwin Vega 6x9s sounded really good. They were a huge upgrade to stock speakers, and were budget friendly. If you get the ones you have listed though you'd want to amplify them. The HED series from Cerwin Vega run pretty well off of stock deck power though and have a really nice sensitivity level. Here's the 6x8s Also Hertz brand, I liked them as well.

Even if you spent $100-150 on a budget-friendly deck you'll get enough power to push the HED Cerwin Vegas much better than a stock year 2000 deck. Not to mention you'll be able to shape the sound much better, it will play much nicer with the subs you plan on adding later, and a thumb drive will hold much more than 6 CDs worth of music. This is what I would do in your place.

Now there are a lot of differing opinions, and you'll encounter the group that will tell you to get really nice components and amp them up front, disconnect the rear speakers, and save up for your next step, and that is an awesome way to go - but it didn't match up to what my budget and priorities lined up with.