You have to decide right away, what your purpose is in assembling this collection.
In my case, I am simply interested in the history, the owning of pieces of history. Knowing that these coins were minted in the lifetimes, and possibly(remotely I admit) in the presence of or being handled by the sovereign on the obverse.
Thousands of these coins were made in the course of a king or queen's reign. The likelihood that they ever laid eyes on any coin you buy is awfully remote. Maundy coins were sometimes handed out by the king or queen at church services during the week of the passion of Christ, and were more likely to have been handled by the actual king or queen...but not necessarily...they had plenty of Lackeys you know.
The other day, I opened a package with Mary I's coin inside. I said to my friend, when he held it: "Can you feel the evil in this penny?" He didn't sleep for two days after I explained what Mary was up to during her reign.
Since I am only interested in the history, I have a great deal of freedom in what I choose to buy. Though I may not get the greatest portrait in the inexpensive coins I buy, I can still feel that history, not just in the perfection of the image, but in the fact that it is a bit "beat up" and has been handled in its long lifetime. Imagine, some of these coins are nearly a thousand years old. Some have been buried for centuries, some have been sitting in someone's bureau drawer for generations and some have been on display in museums. They have probably been stolen a few times, paid for both illicit and legitimate things and services. Their use has led to both joy and heartache.
If you plan to invest in these coins, you must find coins that have not been cleaned in living history. The patina that forms on the coin is very desirable. Gold for the most part does not look much different than the day it was minted, but silver and copper can be black as night after a few centuries. In some cases, silver objects in good condition can be as valuable as gold, because silver does not last like gold does so there are fewer early objects made of silver than you might expect.
Unfortunately, those of you who wish to collect for fun, and to see nice pleasant images on the coins can be greatly disappointed. The good coins can look like little more than little lumps of molded slate, but can have much more value than the nice, clean, shiny coin with a pretty face on one side.
Coins too, can be very affordable if they are a bit out of round. It was a common practice to clip little slivers of silver off the edge of the coins to accumulate silver to be sold or used for casting etc.
This is why so many more modern coins have a patterned edge on them, so that one can easily see when the edges have been tampered with. The coin may not weigh nearly what it did when minted. If I remember correctly, there were 240 pennies to a pound of silver when minted. Start clipping the edges and the total can be well below that weight.
For collectibility, and for maximum value, look for patinated, uncleaned coins with nice unclipped edges. Clean, shiny (unless by some miracle they are that way naturally) coins have little appeal to the big money buying collector.
Look too, at the edges. Some will have an edge that comes to a razor edge, this may be better than one that has a neat squared edge but a little rough, sparkly or flat, as it may be a sign it was trimmed by scissors or snips.
This means, of course: Do not clean old coins except in the most superficial manner!
Handle the coins only by the edges, and cotton gloves would be best, as oils from your hands can etch the metal(you are quite acidic you know). No solvents! Plain water and dry without rubbing.