Welcome Sean. I will chime in with some of my experience and I am sure others will as well.
safe distance as in near brick walls and windows? The Manual says, "Do not use this smoker within five feet of any combustible material." I have mine sitting on a grill matt on a wood deck. It is probably 12 inches from the rail. I am not sure of the official answer here on brick and glass. Probably three to five feet or so. IMHO the issues is not necessarily heat but smoke residue buildup over time. You don't want it to stain those areas.
and if i put under the pergola a safe height distance? Jim has his inside a pergola-like structure he affectionately calls the Pigvillian. I will leave that one to him. I would add that as long as there's good air flow this is not going to be an issue.
are the legs on the WSM stable, as I normally put the weber ketle on the grass or on the garden path!? VERY STABLE. It WILL NOT tip over if on a level surface. However, your grass may not like the heat from the bowl. I recommend a grill matt, or pavers, or something more solid that grass.
& what acceries do I need? This is personal preference. I think charcoal, a chimney starter (with newspaper or starter cubes), a lighter, gloves, tongs and spatula, and a thermometer are the basics. I recommend a thermometer that you can read the cooker temp and one for the meat temp, or one that will read both.
what will it be like in 3 years? I am not sure what you mean here. Some guys have had these cookers for decades and they operate just like new. That's why we select Weber as our go-to brand: they last.
and how much charcoal/heatbeads would say a 6hour pork rst go through on a wsm? That depends on which size cooker you get and temp you're cooking. I have the large 22" but some have the smaller 18". I imagine I use about 3 to 4 pounds of lump for a 6 hour cook at 250* F. When I plan a short cook of 6 hours, I put in a 6.6 lb bag of lump in the charcoal ring, reserve some for the starter, add it back on the unlit pile once the reserved is lit. I don't use all that fuel, but I have it available. When done, I simply close the vents and reuse the unspent fuel for the next cook.
I hope this helps.
