Neither the tables, the computer, a gauge and watch are going to save the inattentive diver. Ultimately, the parameters of the dive are going to determine your needs. A couple of dives to 30-40 fsw, on two tanks with a surface interval in between, you probably won't come anything near the NDL. A SPG will be all you need, unless you are really good on gas usage. If there is the possibility of wandering into deep water, or some other way of getting yourself close the NDL having nothing will not be a big help. Diving on the tables will require a depth gauge and watch of some type. A basic depth gauge will be $70 and a watch will start at $25.
A cheap computer that will track Bottom time and depth and where you are relative to the NDL can be had for around $125-150. It will serve you well and fulfill most of your needs. If you stick with diving, it can be your back-up when you buy something more exotic. A basic computer I think is a no-brainer investment. If you are renting to start, I would put it fairly high on the list of things to get early. First so you can learn to use it properly, second, You will know if it works reliably. Knowing my depth and time during a dive adds to the enjoyment of the dive. I can use the data for navigation, and to track turn points and not just for the NDL.
I learned to dive tables and I use tables for planning, but a computer reduces task loading during the dive because it does a lot of stuff for me (Back in the day, I always forgot to set my watch bevel).