If you decide to go the sound card route, the Signalink USB is certainly a good option and they do turn up at hamfests often enough. These days, the DigiRig and DigiRig Lite seem to be very popular but it hits $100 quite rapidly if you buy their cables. The DigiRig includes CAT control but your TR-751 does not? You could consider it to be "future proofing" but the DigiRig must be purchased (configured?) for CAT to match the mfgr of your radio.
Most of the low cost sound interfaces these days are based on the CM108/CM119 sound chip. You can use any cheap audio dongle and run audio to the radio if you have a serial port available for PTT. There are plenty of audio interface designs that generate VOX-based PTT from the audio input. The WB8LMF web site has a few designs. The cheapest option is a simple "Easy Digi" transformer interface between your laptop and the radio. I have plenty of 600:600 transformers if you want some. Just be certain the transformers are terminated near 600 ohms or the frequency response will be poor.
Another product I found recently is the AIOC board. For $20 it is a USB audio and serial interface for sound card modes. It is perhaps the size of two postage stamps. You could mount it inside the TR-751 and just bring out the USB cable. <
https://na6d.com/products/aioc-ham-radio-all-in-one-cable-unsoldered> There are links on the web page and a number of videos on YouTube that you can review. It is an open-source product and many people are making them.
You will have to decide if you want to incorporate transformer isolation or not. Back when PSK31 sound interfaces were new, it was considered mandatory to isolate the computer and radio to prevent transmitted hum. The Signalink USB has isolation built-in. Many modern sound interfaces like the DigiRig and AIOC do not have isolation. Those mfgrs suggest adding an inline USB Isolator board if you have common-mode hum on your signal.