Lots of great advice from others in this thread. I forget how many trailers that I own (8)? Here is my 2 cents.
1 - axle capacity. More is better. Easy to determine by counting lug nuts. 2,500 lb axle has 5 lug nuts, 5000 has 6, and 7000 has 8. I personally would advise you to buy a trailer with at least a 2,500 lb axle and 15” tires. My smallest trailer (single axle 4x8) has a 5000 lb axle with brakes. Tows like a dream though.
2 - electric brakes are really nice to have, especially if your tow vehicle is not a good sized pickup. Most small trailers wonn’t have them, but if you go with a 6x12 as recommended above be sure to source one with electric brakes.
3 - Steel is fine. Aluminum is nice, but not a “need to have”. As others have stated, aluminum WILL crack over time and getting it repaired properly can be a challenge (and expensive). Steel is nice because it’s easy to modify if you want/need to.
4 - Wood deck is nice. For a small trailer, I’d recommended ground contact rated pressure treated pine over oak. The only reason is weight - treated SYP will weigh 25% less than oak and that’s a few hundred pounds off of your payload capacity.
Personally I try not to move a trailer by hand any more. I simply park it where I don’t have to move it. The ball mounts on a riding more work good for light trailers, as do the roller caddy’s mentioned above.
Regarding grease zerks on axles, I am not a fan of these for anything other than a boat trailer. On a boat trailer overfilling the bearings with grease is desireable in order to keep water out of the bearings. In all other applications an excess of grease can cause overheating and premature bearing failure. We had a long thread about this here on CFF a few years back. Now, other than boat trailers the other exception to this rule are trailers with little bitty tires. Bearing speed is directly proportional to tire diameter. A typical trailer with 15” or larger tires only needs to have the axle bearings regreased every 50,000 miles or 10 years - longer intervals if the trailer is more frequently used and lower intervals if the trailer is overloaded. How often do you regrease the front spindle bearings on a rear wheel drive car? Trailer axles are no different and axles with grease zerks tend to be over-greased. However, trailers with 13” or smaller diameter tires will see more bearing speed / and heat, and routine regressing can push any degraded grease away from the bearing surface which may prolong the life of the bearing.
When you put tags on the trailer, be sure to invest in the lifetime trailer registration. Much cheaper in the long run. I think that the break even point is around 3 years. If you plan to own the trailer longer than 3 years, permanent tags are cheaper.