It depends on what you are doing and what compromises you are willing to take.
I'm still using my dell M6800 and t430 for work, they are decade old laptops.Ubuntu for Os and it's been working great. It's an i7 system and i work on php projects and sometimes dabble in python and some ml projects. Never ever disaapaointed me.
It's been showing it's age when some ml projects throw that the current cuda library is not supported by the nvdia graphics that i currently have.
I buy and sell used laptops and computers parts as my side hustle.
There afew points that you should keep in mind, while buying used laptop.
1.Don't buy U variants or any other low power variants of the i7 processor.
2.Don't buy dual core i7 variants.
3.Try to buy 8th gen or above. Better battery life and performance ratio.
4.If you are not everyday carrying or portablity is a must , stay away from slim and designer ones, that do not have adequate cooling.
5.Check if the display is TN or an IPS panel. TN is a straight no no.
6.Do not buy the models that are targeted towards students and home users, buy something that for professional users. They are more durable and tend to last long. Example lenovo T-series, W-series, HP also has some, Dell M-series, Precision-series.
7.Check the number of output ports.
8.Physical condition of the laptop, if it is kept clean or not. The fans might be clogged, hinghes are loose, some keys might not be working. Some usb ports might not be working or loose, etc etc
9.Reapply thermal paste , even if it's running fine. Factory ones dry out quickly, so you will get the chance to clean up the fans as well
I'm thinking about a low budget system, which will be using
1.X99 D8 MAX LGA2011-3 Motherboard - It has 4 pcie 3.0 x16 slots, dual cpu socket. They are priced around $260 with both the cpu
2. 4X AMD MI50 32G cards - They are old now, but they have 32 gigs of vram and also can be sources at $110 each
The whole setup would not cost more than $1000, is it a right build ? or something more performant can be built within this budget ?