Calculating the resistance of the coil is the easy part - you need to take
the resistivity of copper and the packing density of windings into account.
resistance = resistivity * length / area.
The magnetic circuit is much harder to characterise, many different ferromagnetic
materials are available to form frame and plunger.
Generally the smaller the gap and the lower the reluctance of the magnetic circuit the
larger the mechanical force for a given MMF (magneto-motive force, measured in
ampere-turns).
Go and look up "soft" v. "hard" magnetic materials - you want soft for a solenoid,
with low remanence. Generally iron is the best cheap material, although mild steel
is more commonplace (but less soft in both senses).