If you are running without a sub, you really need to add a small 4 channel amp to output anything worth the effort and money you are going to spend in these old cars. Headunit amps (It doesn't matter what brand or model or price) put out very little actual clean power across the entire frequency range, are usually rated at 1k hz (where humans hear best = artificially inflated) and push a ton of distortion at anything but stop light listening levels in a stock 6cyl car.
Research the features that are must haves for your headunit in quality brands (IE don't buy Boss or some other off the wall brand), read some reviews and then pricematch to Crutchfield. They are very proud of what they sell is all I will say.
I am too many years out of touch to tell you the best bargain options currently, but some things are tried and true for the most part like Alpine, Focal, some Sony, a fair amount of Kenwood and Pioneer, and if you like highs, Infinity is a great way to go. Polk usually offers some quality speaker sets above their bottom baseline stuff as well. I recommend going to hear them on a sound board at an actual car stereo shop, using music that you are familiar with (IE bring a CD you love) and make sure that everything is turned off, or of the appropriate power. Meaning if you don't want an amp, don't listen to speakers with an added amp, insist you listen to them through the headunit only. Or if you are adding a small amp, don't listen to a set with 150w RMS through it when you are buying 50w RMS. MANY speakers sound really great with some clean power, and a large portion of those will sound like crap through just a HU. I won't dive in to the details. Just be sure to audition what you are planning to run with familiar material (IE you know what the vocals or guitars or bass SHOULD sound like).
If you were further north I would help you out myself.
1966 GT Coupe
Not much left original
Trying to do things once
MMI CO / Panhard ~ JRI / Penske ~ Astro A5 ~ FiTech ~ MGW ~ AAW