Many of the answers depend on you and what you want/like, but buy now and regularly to dollar cost average. Buy what you can afford/feel comfortable with while still maintaining am emergency cash fund is what most people say.
What type of silver affects price (how much over spot/melt you'd pay). Ie generic vs Eagles (or other gvt issued) vs 90% etc. Size counts too. Fractionals carry high premiums and sometimes you can get 100 oz bars for spot or under. Exit strategy also comes into play. Buying a 100 oz bar because its low premium may bite you if no one wants to buy it.
If you're in the US I know a dealer in AZ who ships nationwide and can generally beat the big online sites like APMEX, Provident, etc prices. Otherwise local coin shops, eBay, or even Craigslist both work. Be careful on Craigslist though! Meet in s public place etc for your safety. Also, there are a TON of fakes so no matter who you buy from (eBay and Craigslist are bad for this). Make sure the seller can prove the silver is real. A Precious Metal Verifier (PMV) runs $600-800 and basically scans the metal so no need to use acid and mark it forever. Well worth it because weight, markings and magnets mean nothing anymore. Some of the fakes are great! Oh yeah, they're even faking the graded holders too so you think you're safe.
Silver instead of gold? YES. It's comparatively low and can't go much lower (another topic) while gold could easily drop $100 or more easily. However, if offered a 1/10 oz AGE (gold Eagle) or a 1 oz ASE (silver Eagle) for spot, I'd take the gold Eagle because it's rare you ever get them that low. ... knowing that comes later though...
I hope this helps! I'm always happy to help answer, teach, clarify or whatever.
Mahalo
RE: Buying Silver