Let's talk about Step-By-Step for setup you are going to buy with some detailed explanation. I'm not going to compare any parts with my older build provided up.
1. Cabinet : If you like Thermaltake Ceres 330 cabinet. It's all your choice as it doesn't matter at the end you are going to show-off it to yourself or gonna watch it while doing work to feel relaxed
2. Processor : AMD Ryzen 7 5700X |
Intel Core i5 - 12400 | You can choose one of them from both, Although Intel is better in Multi-Task working but you can go with AMD too. At the end of day, It depend on you which one to choose.
3. Graphic Card : RTX 3070 8GB (Supposing 8GB Card) , I'm not sure whether this card will be best because as you've said that it is 2nd-hand and I'm not fan for going to get 2nd-hand Graphic card as it can get any issue at anytime in future and you'll have to pay for double the price for parts of Graphic Card to get replaced.
I can't say any particular tests where you can test it because it takes time to notice defects on Graphic Card and Mostly in the Market, Miners sells there Graphic Card which will show many issues while using it after 3 - 4 weeks of usage. (self-experienced with 3060)
MSI GeForce RTX 4060TI X3 Ventus 8GB OC is better than MSI GeForce RTX 3070 8GB both in performance and Avg. FPS Gaming.
But If you want to get RTX 3070 (2nd-hand) it's all your choice. At the end of day, You'll be the one which will be going to use the PC and should've satisfied relief by yourself.
4. Power Supply Unit (PSU) : Asus ROG Strix 750W 80 Plus Gold SMPS | Perfect for your PC. Don't bother about PSU and choose this one.
5. CPU Cooler : ARCTIC Freezer 34 eSports DUO | For AMD or Intel below 7th / 13th Gen, It is perfect so you can OPT for this CPU Cooler. Choose Color matching with your Cabinet for better looks and aspects.
6. Primary Storage : XPG S70 Blade | Perfect in market and usage. I've been using it myself and it is 3rd - 5th Fastest
SSD so doesn't need to care about other aspects. Choose for it.
I would recommend you to get 512 GB or 1 TB Extra Secondary Storage for General Purposes. (Prefer NvME or SSD over HDD)
7. Random Access Memory (RAM) : Don't buy DDR4 RAM sticks. Choose DDR5 RAM Sticks as it'll be more suitable for your build and working for Unity/Other applications.
Change your RAM Preference and get DDR5
My Recommendation : Adata XPG Lancer 16GB DDR5 5200MHz or 6000MHz (2x16 GB Stick)
Note* : AMD Ryzen 5700X doesn't support DDR5 RAM so, You'll probably choose DDR4 and If you are going to choose between 1 of them
8. Motherboard : You'll be going to get DDR5 Motherboard, So you'll have to get DDR5-Supporting Motherboard
Choose this one : Asus ROG Strix B550-f Gaming (DDR4-Support)
9. Monitor : Acer XZ306CX 29.5 Inch (74.93 Cm) Ultrawide 21:9 Curved 1500R | Perfect for you if you want to choose it.
Otherwise,
Samsung Odyssey G4 27 Inch (G-Sync Compatible) if you are interested in doing Gaming or DLSS supported games. Otherwise, You got a decent Monitor from Acer.
For any other parts changing, You can do it yourself. I'm more caring about your 2nd-hand Graphic Card than any other parts in your build. 
