I've been running BIAB for more than 25 years, on so many laptops I can't count, here is my 2 cents on what you might want to look at before shelling out $$$

1. i7 processor. I would say at least 2GHZ min.
2. Min 8 GB of RAM, even with 64 bit Win install you don't really need more.
3. Many people say get Win 10, but actually if you get rid of all bloatware and strip it down to use minimum resources, Win 7 64 is amazing.
4. An SSD drive these days is a no brainer, it has significant advantages over a 7,200rpm platter drive, it will run cooler as well. I highly recommend the samsung 850 EVO drives, 512GB will go a long way if you keep things organized.
5. ASUS, DELL, HP all have some great used models (within 2-3 years old)
6. Ccleaner, AVG free, Malwarbytes all have free versions to keep your laptop running in top condition.
7. Be sure to upgrade to BIAB 2017, the generation of RTs is much better.
Good luck, there are some fantastic buys on used laptops on ebay and craigslist.