I would not recommend it, unless you have a problem with your budget. For one thing, it is rather warm to sleep on a memory foam topper. For another, in cold weather, it can take 10 or 20 minutes for your memory foam topper to warm up enough to mold itself to your body for maximum comfort. So if you have the budget for it, it is probably better to buy a proper bed or mattress that is right for you.
That’s not to say buying a firm bed with a foam topper is no good. I’ve done it myself, when my company assigns me to locations where I cannot get a good bed. I have spent $50 to $100 for a cheap and firm mattress with a separate foam topper (not memory foam) laid over it. This combination has given me some decent sleep, especially when I was younger and fitter. Now that I’m older, it feels somewhat lacking.