I would like to work with the Ethereum blockchain and be able to get transactions associated with an address. I believe I cannot use the fast sync mode and need to have a fully valditating node?
Do you have advices for the AWS specs?
Do I have to maintain an index to get list of transactions like https://etherscan.io/address/0x281055afc982d96fab65b3a49cac8b878184cb16
or it possible to access it directly?
No, you can actually use the fast syncmode and you will still be able to get transactions associated with an address using events. Here's a similar answer.
I have tested working Geth with option cache=4000 on t2.large (with 8GB RAM) and the CPU usage is around 50% until the node synchronizes completely, after which the CPU utilization is fairly low. You should be concerned more about the storage I think.
Answering your third question, it's mainly a software design and need-based issue. If you want to open an API route that's going to make quick requests, or if you're tracking a lot (or say all the addresses), then it'll be a better idea to 'index' the transactions. Etherscan can't do that on the fly requesting the node for tracing the transactions either. However, if you do not have a huge traffic/throughput requirements, you don't necessarily need to index the transactions. Think about it that you'll have to maintain an almost the same size of database to index the transactions.
