I have 7x14 Microlux and a 7x10 Harbor Freight. Unless you really need to save money, avoid off brands of import metal lathes. Unless you really really like to tinker with taking the lathe apart, tuning it and putting it back together go with a name brand.
I bought the Little Machine Shop DRO add-on for the mini lathe and they work great. I can't imagine using the mini-lathe again without the digital read outs, they make life so much simpler.
If I had the budget and a do over I would have bought a lathe with the DROs preinstalled from Little Machine Shop. They seem like good people and have quickly answered questions for me in the past.
IMHO the big question is "What material do you want to turn?". There is a big difference between turning Titanium and turning Acrylic. The next big question is "how much time do you have?". You might be able to get by turning harder materials on a smaller mini lathe, but it will probably take you longer and you'll have to spend more time adjusting/modifying the machine. Then "What else might you want to do?", the sky is the limit on this one, but it is usually easier to make smaller items on a bigger lathe than the other way around.
Used old iron is one way to go, but you will usually need 220V or need to convert to 110. A lot of these are on the "way too big" side, but some are decent and come with necessary tools. But this isn't the route for the faint of heart (as far as I can tell).
Here is what I suggest considering:
Budget - this can narrow things down quickly. The options shrink pretty quickly under about $2000. Unless you are going the used route.
Minimum size 7x14, or so for penturning. Once you start adding chucks and stuff you can run out of real estate faster than you expect.
If you are really wanting to do harder steels like stainless or stuff like titanium then I'd suggest moving into the 10X20 class or larger. These materials can even be a stretch for the largest 110V machines (12x30 or so)
Variable speed is pretty necessary for this kind of stuff, but this is also in the "how much time do you have?" category
110v or 220 once you get up into the 14X40 class basically all are 220V. Also, the larger gearhead lathes tend to be a bit louder
Consider tooling availability and compatibility with what you have. Chuck mounts are not standardized below the 12". Also, once you get up into the 12X20 class the tailstocks tend to be MT3 and the headstock MT4 or 5.
Space is also an obvious consideration
Lathes tend to be pretty heavy. Once you get into the 12X20 class you may not want to haul it up or down stairs
I probably didn't help you narrow things down much, but hopefully gave you some food for thought
A 7” mini metal lathe will work for pen making. I have a 7” x 10” HF and have used it for over 4 years. It works. In hind sight a 7 x 14 or 7 x 16 would have been better if you have the money. It gives more room to drill.
I’ve been able to turn O1 tool steel, 300 and 400 series stainless steels, even hardened SS Damascus requiring carbide tooling on my little lathe. Aluminum, brass, copper, bronze too, using the right tool geometry and a good cutting fluid. Tooling for the smaller lathe is also cheaper. MT2 tooling fits my wood lathe too.
I’d love to have a 12” or 14” gear head lathe one day. But I’ve learned a lot using my $400 HF lathe and made some quality pens in the process.
I’ve put off buying a mill for years ratcheting up the size and spec of what I “need” to a price point I end up putting off the purchase. Glad I didn’t go that route with the little lathe. Make a budget, half for the lathe and about half for tooling. Be realistic with what you’ll use it for. Pull the trigger and spend time turning. Make sure the model you choose is well supported for parts and find a community of users for tips and upgrades.
I have a Grizzly 10x22. It is nice, and has been good for me. I have it on a double wide tool chest. Its handy because the tooling is right under the lathe. I am still very much an amateur, but it is a great time learning. I also have wood lathe so I can shape pens on it. I fashioned a tool rest for the metal lathe but it still seems unhandy. The metal lathe is variable speed, but has gear changes to effect that range. Changing gears is a giant PITA, so I have the range set for about 600 to 1700 rpm, which I feel is to low to use lathe chisels, but it certainly can be done. So I move pens back and forth some times. Having said that Danny only has his metal lathe and makes some pens that we all drool at. He is a good machinist and has helped a lot of us at times.
My metal lathe is mt 4 in the head stock and MT3 in the tail, which has led me to purchase all new stuff. There are adapters but they are not as easy as they would like you to believe to interchange. While I really like the 10x22 size of the machine, it has had it's challenges for tooling, being as my wood lathe is MT2. There was some expense in duplicating some tools.
I was at a friends house and he had a LMS lathe. While it was a fair amount smaller than mine, it was an awesome machine. It had both x and y power feed. THey are pricey for the size and the chip tray is extra, where most companies include that, but it sure was nice. It just seemed more refined than my lathe.