Imac 7,1 (2007) software update from Snow Leopard (10.6.8) to El Capitan

Hello all,

I've been trying to update my parent's Imac (2007, 20 inch, 7,1) model from Snow Leopard to El Capitan but am encountering errors and conflicting information online.

I have upgraded the HDD in the computer to an 8gb/s Samsung 850 Evo SSD which is running well. In addition, the computer has 4gb of RAM already installed so as far as I am aware the computer certainly meets the minimum specs for El Capitan. In addition I have read elsewhere that the 7,1 model should be able to support El Capitan. However, when I attempt to upgrade the software off the apple menu it indicates that no updates are available. In addition when I try to manually download the operating system off the app store through the apple website link it says "we could not complete your request // There was an error in the App Store. Please try again later." followed by "Software Update Required // To make changes to your payment information, you need to upgrade your Mac to the latest version of macOS."

I'm now unsure of how to proceed after reading the forums so far. I have made sure I have the up to date version of Snow Leopard (with the patches for the appstore) and although I'm not 100% certain I think this computer should be able to cope with El Capitan. Anyway any advice would be most welcome!

p.s I was considering updating the RAM on this bad boy as it is. I Believe 6 GB is the max this model can utilise if anyone has some advice on this, ie is it true is it worth it etc. that would be much appreciated!

Many thanks.

Model Details:

Model Name: iMac

Model Identifier: iMac7,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 4 MB

Memory: 4 GB

Bus Speed: 800 MHz

Boot ROM Version: IM71.007A.B03

SMC Version (system): 1.20f4

Serial Number (system): W8*****X87

Hardware UUID: 00000000-0000-1000-8000-001B63A27341

[Personal Information Edited by Host]