I remember that Kit. I built one when I was around 15 back in 1975. I am returning to model building after a very long hiatus. It's been almost 40 years. A lot has changed since those days. You're doing a great job.
Since returning to the hobby about 2 1/2 years ago I started using Acrylics almost exclusively. I use lacquer primers and varnishes and I also use acrylic varnishes. I have some acrylic primers but haven't used them yet. I still have some enamels but rarely use them.
For the best results you should always wash the sprue trees in a warm water with dish soap to remove the manufacturing oil residues. I went to Walmart and got some of the inexpensive aluminum food trays for parties some Large ones and some smaller ones. I use the large ones for cleaning the kit sprues and the smaller ones for cleaning my airbrush. That way I don't lose any parts.
I recommend priming regardless even though enamels and lacquers still bond its a good habit. A couple of thin coats of lacquer primer will give you a good foundation.
Also if you intend to use any washes and/or weathering you should do a clear coats prior to protect your work and after when you like how it came out. I learned the hard way and had to strip all the work I did and start over and that was a 75mm figurine and it really sucked.
I experimented with Vallejo, AK, Life Color, AMMO and now AMMO's new ATOM line. I really like the new ATOM line it has a decent consistency. Overall using water to thin can leave a chalky appearance. Definitely use the thinner/retarder. You won't get the clogging issue like you would get when using water and personally I like it for hand painting as well especially if I am going to do some blending when painting figurines. I know it's not a military figure but I just finished this which was a gift (or maybe a curse) from my daughter from a year ago. Painted with Vallejo, AK, and AMMO.
Vallejo's Game and Military Colors are a mixed bag when it comes to consistency. Generally the highlight and shadow colors tend to be a little too thin while the base is on the thick side. Their Air Color line is pretty descent consistency wise and like some of AMMO's paints can be airbrushed straight from the bottle. I still recommend using the thinner/retarder though. You'll have to experiment but as a rule 1 or 2 drops mixed in the cup generally works. Just make sure to use their thinner and not another brand.