I have two of these Orico adapters and they fit perfectly (2.5" SSD in to the adapter, and then the adapter in to the 3.5" factory bracket) and are cheap ($7 on Amazon). https://www.amazon.com/gp/product/B01LZWX6PD/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://blog.greggant.com/posts/2018/05/07/definitive-mac-pro-upgrade-guide.html
It's a long read, but to answer some of your questions:
I'm running 6 sticks of this RAM https://www.ebay.com/itm/152735388752 others may post better deals though.
An SSD would be wonderful. And adapter like this will let you use it with your sleds. Also, the Mac Pro has SATA 2, which is only 300 MB/s, but it's still a 10x step up from a mechanical drive and there are ways to improve things further if you want to.
As for which SSD to use, I have Crucial and PNY, they work fine.
Do you know if your machine has 1 or 2 CPUs already? Swapping to a 2 CPU set up would be nice, but it's not cheap and you need to make sure it's a 4,1 tray even though your machine is now a 5,1. Also, swapping CPUs in a dual tray 4,1 is more difficult because they need to have the metal cap removed (it's called a heat spreader). You can do it yourself, but you run the risk of breaking the CPU.
Hijacking el primer comentario para resaltar algo que aclaró u/terryheavy
El precio de de 6,39 es con descuento Prime Member. En la misma captura de pantalla del site de Amazon abajo aparece que con membresía Prime "Ahorras 1,60". 6,39 + 1,60 = 7,99.
De hecho, busqué el producto que dice OP en Amazon y me tiró el precio US$7.99 + US$ 45.88 envío (Únete a Prime para ahorrar US$ 1.60 en este artículo).
https://www.amazon.com/-/es/ORICO-adaptador-interno-Converter-interfaz/dp/B01LZWX6PD
OP trabaja para la competencia y quiere difamar a Tiendamia? Yo trabajo para Tiendamia? Es una simple confusión? Guess we'll never know
edit: gramática
I got ORICO 2.5 SSD SATA to 3.5 Hard Drive Adapters that someone here recommended for $6
I like this price better than that of the OWC AdaptaDrive bracket. Too bad I bought three a few weeks ago . . . on Amazon too.
My next project now will be to see if I can (and want to) get up to Catalina... have more research to do!
If I were you, I would look into OpenCore.