Interestingly, I have just been buying vintage laptops on eBay for this exact scenario. I bought early Win95 and Win98 era laptops as they tend to come with USB ports in addition to the serial and parallel ports. These laptops use hardware serial and parallel ports that do not cause the issues sometimes found on more modern ‘virtual’ serial ports that are in fact USB Serial bridges.
If you go earlier than the Win95 era of laptop you can lose USB ports, CD drives and sound support. I have some 386 and 496 powered laptops running DOS and Win3.1 but they can be a pain to use as any audio or CD support has to be attached to the Parallel port, PCMCIA port or dedicated expansion connector.
It is a good idea to have at least one well made laptop from the Win95/98 era that offers the usual legacy ports and a CD drive as you can run it in Win95/98 or pure DOS mode for software compatibility purposes. Sadly these vintage laptops have become popular with hard core Retro Game fans and prices can be surprisingly high on eBay. I bought faulty units and repaired them as a more cost effective solution
Be careful buying vintage laptops at low prices that are ‘untested’…. Many may be beyond economic restoration to working condition. Most need a RTC battery as a minimum and the Ni-Mh cells pack I used in Toshiba units are quite expensive. The main battery should be considered ‘end of life’ but is not needed for desktop use… just be wary of Ni-Mh packs leaking electrolyte onto the motherboard !