Most current devices that OpenWrt supports can use the mwan3 software.
Older devices with only 4 MB of flash probably don't have enough space available. Some devices with only 8 MB of flash may be challenging as well. These older devices are usually MIPS-based (the computer architecture of the "CPU" or "SoC") and don't have a lot of processing power, especially for lines over 100-150 Mbps.
If you do not already own a router, one with at least 16 MB of flash and at least 128 MB of RAM1 is recommended.
If you have more than around a 100 Mbps line and want to use SQM or VPN, a multi-core, ARM-based device is likely a better choice than a single-core MIPS-based device. These units start somewhere around 75€ for new units, less for used or refurbished.
1 At least 256 of RAM if it has multiple, "ath10k" radios, which mainly rules out a couple of Asus devices.