To begin a discussion of dumbbells, I have to start with a disclaimer. I cannot possibly know all of them by experience – only by observation and reading notes that the producers give us. Your options are extensive. ‘Traditional” or “New” characteristics make the first dividing line. The “Old style’ is made with the same plates that are used for barbell loading, Commercial gyms usually have an extensive collection of preloaded dumbbells in weight increments from 5 pounds to 70. A small collection starting at 5 lbs would be one choice for you. They can be bought adjustable as well. The adjustable ones have threaded ends on 12-inch handles. Threaded collars secure the plate combinations you choose, These two kinds were all that was available for decades until recent years. They work well and are inexpensive. A more inexpensive type is made using water or sand as the controlling level of weight change. The obvious possibility of leakage makes them unacceptable in my view and I will say no more about them.
The traditional kinds have advantages that should be mentioned, however. The adjustable ones are relatively small In your hand. Even a 30-pounder would only have 2 plates on each end of the dumbbell (10 & 5). That they are round means they can roll – enabling several gut and chest exercises, though quite advanced. That the plates fit traditional barbells is convenient if you want a barbell or have one. Cost is the final advantage. They are cheap. Here are disadvantages: changing the weight on your adjustable pair takes a few steps: unscrew the collar on either end, change the plate load, screw the collar back, now move the loose plates out of your way. The supply of loose plates must also be stored somehow when not in use.
Sets of nonadjustable dumbells would be a modern option with a few styles to choose from. A set would include 4 pairs at levels 5 lbs apart – or 7.5, or 10 lbs apart depending on your strength and progress. For example, a light set would be a pair at 5lbs, 10 lbs, 15 lbs and 20 lbs. As you get stronger you can buy 1 pair of 25’s. They do not come adjustable but you can adjust the weight by adding ankle straps at 2.5 or 5 pounds. Storing a set is not a problem and it becomes a convenience not to need adjusting – switching from one to the other quickly All types in this style have an octangular round ball at each end of the dumbbell and knurled grips. They are so similar that price becomes the only determining factor in choosing. They are what I use at home and I am pleased with them.
Modern adjustables have some very big advantages. One pair will cover a range of different weights saving a huge amount of storage. To adjust, you simply move a dial to the number you want and lift the changed dumbbell from its rack – very, very quick and easy. The main disadvantage is in the cost which is enormous – the highest. The other disadvantage is in the size the length of the bar to hold all of the plates at the largest size. The problem is that the smaller weight has a bar that is awkwardly long. Any dumbbell holding over 20 lbs will be a big dumbbell but little ones that are nearly 2 feet long will be awkward to handle. There are several versions of these adjustables that use a cradle for containing the big one assembled. They allow a quick change to lift out the size you want. It is the method of changing that varies but speed is achieved with all of them. My favorite is the REP Quick draw version because it overcomes the problem described above for the bulky small dumbbell.
Amazon’s very extensive collection will allow you to choose optimally.